Power nap timer

Twenty minutes is the usual recommendation, and the reasoning is about sleep stages rather than a round number. You spend it mostly in lighter sleep, so waking is easy.

Somewhere past about thirty minutes you drop into slow-wave sleep, and being woken out of that produces the groggy, worse-than-before feeling called sleep inertia, which can take half an hour to shake. That is why a long afternoon nap so often backfires.

The other option is a full 90 minute cycle, which lets you come up naturally at the end. There is not much good in between.

Or set any length you like: gotimer.app