TL;DR When’s the ideal time to drink coffee? How many minutes before caffeine kicks in Those are valid questions. In fact, caffeine takes some time before it goes into full gear. And there are a number of factors that make caffeine stay longer in your system.
If you’re one of the many coffee-drinkers who regularly drink a cup of joe for the morning jolt, then you must’ve wondered at one point:
When’s the ideal time to drink coffee? How many minutes before caffeine kicks in?
These are valid questions. In fact, caffeine’s nothing like Popeye’s spinach that works the moment he pops that can of power.
Caffeine takes some time before it goes into full gear.
So the million-dollar question is - how long does it take for caffeine to work?
That’s exactly what we’re going to find out in this article.
You might feel drowsy and incapable of functioning at a high level until the caffeine hits your bloodstream. You may go from 0 to 60 in about 10 minutes, but you hit top speed after about 45 minutes, when 99 percent of the caffeine from your coffee, tea or energy drink has entered the bloodstream fully. Then it takes many hours for the caffeine to dissipate.
These findings were reached by researchers from the University of Barcelona in a study published in December 2008 in the journal Progress in Neuro-Psychopharmacology and Biological Psychiatry. 
The 10-minute finding contradicted previous studies that showed alertness does not increase until after 30 or 45 minutes, according to an article in Science Daily. 
To measure caffeine’s effects on the human body, the researchers analyzed 668 university students, 238 of them male. Their average age was 22. They sampled the levels of caffeine in their blood at 10, 20 and 30 minutes. They took the measurements two times during the day after the students drank coffee, at midday and in the late afternoon, to serve as a control if there were possible differences caused by the time of day.
“Forty-five minutes is the time needed for maximum caffeine concentration to be reached in the blood, but levels reach half this concentration after just a few minutes,” Science Daily reports.
To cut the long story short, the magic number based on these reports appears to be 10-45 minutes. During this period, you’ll feel caffeine’s effects in full steam, like an energy boost, optimal mental performance, enhanced sports performance, a happy mood.
Or if you have too much than your body can usually take, you’ll also get the adverse effects - anxiety, depression, jitters.
Afterward, the liver metabolizes caffeine and out it goes from your body.
So if you have an important presentation or an exam, then better have that first pop of caffeine mints or a cup of java 10-45 minutes prior!
Now that we know how long the effects of caffeine are felt after that first sip (or mint pop), the next question is how long caffeine actually stays in our system.
The University of Nottingham School of Health Sciences defines half-life as: 
the duration of action of a drug [and the] period of time required for the concentration or amount of drug in the body to be reduced by one-half.
We usually consider the half-life of a drug in relation to the amount of the drug in plasma. A drug’s plasma half-life depends on how quickly the drug is eliminated from the plasma.
While half of the total caffeine dosage consumed get cleared from the body in 5 hours for most people, it normally takes 8.25 hours to 2.18 days to completely rid it of the stimulant.
How long caffeine stays in your body depends on several factors.
For adults, the half-life is usually 5 hours. For babies and kids? It’s 80 hours. Big difference, right?
Here’s something interesting - the caffeine metabolism of senior individuals aged 65 and above of 1.0 ml/min is 33% lower than most adults’ 1.5 ml/min.
This may sound like common sense and not that hard to imagine.
The bigger, taller and heavier an individual is, the faster the rate of caffeine metabolism is. Having more body fat can help make the metabolism and clearance of caffeine from your body faster.
The effects of caffeine and the body’s ability to metabolize it may differ based on genetics. Some people are able to process caffeine 40 times faster than others. That applies to the rate by which the body goes through caffeine clearance. [9, 10]
If you drink coffee or tea on an empty stomach, you’re more likely to feel its effects faster than when you’re eating a bagel with it. If you eat fiber-rich food, then it’ll take some time for caffeine to go full blast in your system.
And because of the impact of food on caffeine absorption, the same may be gleaned with how rapidly the body can clear itself off caffeine with and without food.
People with a healthy liver and kidneys tend to metabolize caffeine at a normal rate, with a half-life of 5 hours. But those with impaired liver and kidneys will find it harder and slower to break down the caffeine.
According to studies, those with liver dysfunction will have a caffeine half-life anywhere between 60 to 168 hours - a massive spike from the usual 5 hours! 
Kidney impairments may also lead to slower metabolism but not to the same extent as with unhealthy liver.
Men obtain more of the stimulating effects of caffeine than women. An article by Science Daily reports: 
“Although both the men and women saw an improvement in their activity levels with the coffee, which increased in later measurements, we observed a greater impact among the males.”
One reason why this is the case? Some women take oral contraceptives, which can slow down caffeine metabolism by an extended 3 hours. 
Some drugs can also accelerate the breakdown of caffeine and clearance from the body. Others can keep caffeine a little longer than usual.
Because caffeine is broken down in the liver with the help of CYP1A2 isoenzymes, substances that interact with these also affect the rate of metabolism and clearance.
Those who take drugs that induce CYP1A2 will experience faster clearance. These inducers include Insulin, Modafinil, Nafcillin, beta-Naphthoflavone, and Omeprazole.
Read this article to find out whether you can combine caffeine with certain medications.
On a similar vein, smokers tend to have a quicker clearance rate than non-smokers with 155 ml/kg/hr compared to non-smokers at 94 ml/kg/hr. In fact, smokers are reported to have faster caffeine half-life with 2.5 hours!
The more you have caffeine, the slower you get to metabolize and excrete it.
Just by the sheer amount of caffeine in your body will make it take a little longer to complete the metabolism.
According to Mental Health Daily: 
A frequent coffee drinker may consume several cups throughout the day, resulting in ~300 mg of ingested caffeine. After the third cup of coffee, the body will not have fully metabolized nor excreted the caffeine content within the first two cups.
As a result, a greater total amount of caffeine metabolites (i.e. paraxanthine) will have accumulated throughout a frequent user’s system. Greater accumulation of caffeine may result in a higher degree of reabsorption prior to elimination, ultimately prolonging clearance times. Furthermore, since the frequent coffee drinker may ingest their final (third) cup of coffee later in the day (e.g. afternoon), it won’t be metabolized nor eliminated as quickly as someone who had just one cup in the morning.
Another easy explanation here -
The less caffeine you take, the quicker your body breaks it down.
According to the same article on Mental Health Daily: 
An extremely low dose of caffeine would be within the 10 mg to 20 mg range. This range is just enough to feel its effects, and the body shouldn’t have any difficulty efficiently metabolizing such a low dose. Most people consume around 200 mg of caffeine per day, resulting in slower metabolism compared to an individual ingesting a lower dose such as 10 mg.
A heavy caffeine consumer may ingest over 400 mg per day (equivalent to 4 cups of coffee). At this point, enzymes in the liver may be overtaxed and more caffeine (and its metabolites) may accumulate within the body. This accumulation may prevent efficient clearance and result in reabsorption, prolonging excretion times relative to dosage consumed.
We have good news for lovers of America’s two favorite mood-altering substances, caffeine and marijuana: Caffeine may enhance a pot smoker’s high.
The downside is that caffeine can also increase the chance of becoming addicted to cannabis.
If you often find yourself going to a room in your house only to forget what you’re meant to do, or talking about a movie character you’ve seen so many times but can’t remember the name of, then one thing is going on.
It’s so easy to dismiss these as simple moments of forgetfulness. But if this happens a lot, then it may be your body sending you a signal to do something and beat brain fog.
The writer, Dan Charles, bought a 4-ounce bag of caffeine and said it had as much caffeine as 1,000 tall Starbucks lattes. He said this caffeine was created in coffee beans on a hillside in the tropics. “Slowly and quietly, driven by the energy of sunlight, it formed inside coffee beans hanging on thousands of trees, most likely in Brazil or Vietnam,” Charles wrote.
“Those beans were harvested, loaded on ships bound for the port of Houston, Texas, and ended up at a factory within sight of downtown Houston: Atlantic Coffee Solutions. It’s owned by one of the world’s largest coffee traders, ECOM Agroindustrial Corp., which is based in Switzerland.”