The half-life, which is the time required by the body to metabolize or inactivate 1/2 of a substance activity, for caffeine (doses lower than 4.50mg/lb) ranges from 2.5 to 10hrs. Meaning, caffeine has little to no pharmacological effect after this time but may still be present in small amounts in the body. The variability in rates of elimination is due to an individual’s efficiency in metabolizing caffeine, not absorption. The modern average cup (8oz) of coffee may contain 100-200mgs of caffeine so its effects should be inactivated within 2-5hrs depending on individuality. But it may take the full day to eliminate all traces of caffeine from the body.