Before 11pm for my 3yr old because that's when i go to bed. Kids adjust fast though. My 2 bigger boys used to stay up late because they didn't have to get up for school. 2 days of waking up early for school, they were ready to fall asleep by 9pm after the long day.
It's so hard! My son went to bed really late. It didn't change until we traveled back from the east coast and he got onto an earlier schedule. I heard if you do want to adjust bed times, to do it 15 mins earlier or later a night.