He goes to bed when he's tired. He wakes up at different times, depending on what we are doing in the day - and only has a nap somedays.
We keep a watch on his behavior, and when he starts getting clingy or unco or silly or repetitive we tell him "When the big hand gets to (whatever number is in 15 minutes) it's get ready for bed time".
Some nights that's 7.30 (earlyish start, no nap) other nights its midnight (slept in and had a nap).
We also sometimes give him the choice mid-afternoon "are you going to have a rest or are you going to bed straight after tea".
It works pretty well for us.