I love naps, but I have to be careful with it or sleep will be bad.
Ideal: 25 minutes, and that is after tinkering between 10 to 45 minutes. 25 is enough to get calm, clear headed, and rested without waking up groggy.
The worst is to take a nap and have it to be worse than not taking, usually longer naps do that. I think vox actually had a good video a few years back about it, and that time is also about right if you slam coffee/caffeine right before and then take the nap because it hits your system right as you wake up. Haven't done that often but it does work.