I recommend napping for either 20 minutes (as this decreases the total amount of NREM sleep when you go to bed for the night, as in, you will dream faster), or for 90 minutes.

The latter option is even more preferable, and necessary if your goal is to WILD/DILD during your nap. I haven't had a lot of lucid dreams from naps, but that's probably because I don't nap often. But when I do manage a WILD, I end up with a lucid dream of god-tier quality.