Whenever he falls asleep. Not being snarky, but we don't have a bed time. DS is 4.5 -- we always just followed his lead. If he looks like he's melting, we try to start in the general direction of PJ's, teeth brushing, etc. Usually he just passes out on his own (laying in bed with DH and I, playing or reading with DH, laying next to me on the couch while I work on the computer, etc).
In the winter he tends to fall asleep earlier (darker earlier) summer get rather late as we are outside playing usually until 9pm or so.