We were having the same problems with our DS, and found, that he could have up to 2 hours nap during the day, as long as it was not past 2pm.
He will then go to bed at 8pm and usually wakes anywhere between 7:30ish and 8:15ish. So we generally have around 12 hours at night.
Sometimes he will only nap for about an hour at nursery, and then we find that we will have to wake him in the mornings.
Depending on his mood with a shorter nap, we will either have a longer wind down routine, or some extra quiet time in the evenings before bed - we find on these days he usually falls asleep within about 10 minutes where if he has napped well, he will take up to 1/2 an hour to fall asleep.
Hope this helps.
Paula x