The general rule here (Quebec) in the centers is that naptime is 2 hours. Everyone lies down. If a 4 year old or older does not fall asleep within 1/2 hour, they may do something quiet (books, puzzle...). I follow the same policy, although I may make them try for more than 1/2 hour. I have yet to have a dck not nap on a regular basis. Here they start kindergarten at 5, so it is likely to happen next year.
That being said....my own daughter has not napped for over a year. shhhh Noone's caught on yet