…is about 4 hours.
The data vary, but for a medium-sized adult you can probably expect half of the caffeine from that 3pm cup of coffee to still be doing half of its thing about 7pm, 4 hours later. Apparently smokers deal with caffeine quicker and caffeine sticks around in children much longer.
You can expect about 10 minutes before it first kicks-in which, I suppose, is why if you’re feeling tired while driving it’s a decent plan to take a break and drink a coffee, then have a 15 minute nap. See tiredness can kill, take a break. Once it starts to kick-in you might hit peak caffeine any time from 45 minutes to 2 hours later.
Caveat: that chart visualisation is extrapolated from reported data, so the shape is approximate only.
Some references say longer, and some say, from 3-9 hours or so. My source which seemed pretty comprehensive was:
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