A typical 8 oz cup of coffee contains about 95–100 mg of caffeine, which takes roughly 10–11 hours to fall below the level that disrupts sleep (~25 mg). Use the calculator above — enter what you drank and when for a precise answer.
Caffeine has a half-life of about 5.5 hours in healthy adults — meaning half of it clears from your bloodstream every 5.5 hours. A 100 mg dose (one coffee) takes about 10–12 hours to drop below the sleep threshold.
At the average 5.5-hour half-life, 100 mg of caffeine (one cup) stays above a sleep-affecting level for about 11 hours. A double espresso (~130 mg) takes about 12–13 hours. Genetics, medications, and pregnancy all affect the rate significantly.
The half-life of caffeine is approximately 5–6 hours for most healthy adults (this tool uses 5.5 hours, per Nehlig et al., 1992). Smokers clear caffeine faster (~3 hr); pregnant women significantly slower (~15 hr).
Coffee itself doesn't have a half-life, but its active ingredient — caffeine — does. The caffeine half-life is about 5.5 hours. After one half-life, 50% of the original dose remains. After three half-lives (~16.5 hr), about 12.5% remains.