https://sleepeducation.org/sleep-caffeine/ Caffeine begins to affect your body very quickly. It reaches a peak level in your blood within 30 to 60 minutes. It has a half-life of 3 to 5 hours.

https://www.ncbi.nlm.nih.gov/books/NBK223808/ The mean half-life of caffeine in plasma of healthy individuals is about 5 hours. However, caffeine's elimination half-life may range between 1.5 and 9.5 hours,

https://www.news-medical.net/health/Caffeine-Pharmacology.as... The half-life of caffeine (time taken for the body to eliminate one-half of the caffeine) varies widely between people, depending on factors such as age, body weight, pregnancy status, medication intake and liver health. In healthy adults, the half-life is approximately 5 to 6 hours. Heavy cigarette smoking can decrease the half-life of caffeine by up to a half and in pregnancy, the half-life may be increased by as much as 15 hours. [...] The stimulatory effects of caffeine may begin as early as 15 minutes after ingesting the drug and last as long as six hours.