Confidence votes 821. There're 90-105 mg of caffeine in 8oz of coffee. The body retains up to 300mg at any time. Of course that depends on how fast you drink the coffee, and how fast you can detox it. Depending on the person, it would last in the body 4-6 hours.fter someone drinks a cup of coffee or tea, it only takes about 10 to 15 minutes for them to start feeling the effects of the caffeine in their system, such as being more ale … rt and awake.