My DD is 3 3/4 and has a two hour nap in the afternoons (from about 1pm-3pm). She doesn't nap at preschool (two days a week) and is often so tired the following day that she'll sleep over three hours. She goes to bed 7:30-8pm and is awake around 6:30-7am.

I also look after another boy (same age) every Friday, and while his Mum has little success getting him to nap he goes down without fail at my place for two hours, same as DD. Doesn't seem to affect him going to bed that night. He will do it some days for Mum too.