I aim for 9 because *I* am not a night owl, but it's my only real chance for me-time. If they're actually asleep by 9:30 and I actually make it to 11, that's a whopping 10.5 hours to myself (or shared with dh) a week.
Sure, there are times they stay up later. But you should see me after a few consecutive nights!
Whenever they go to bed, the 7yo is usually up between 6 and 7, the 3yo not far behind, and the girls could stay in bed until 10 if I let them. When the teenager is home, he pretty much sleeps whenever he wants, but I still reserve the right to banish him from my space at any point after 9pm, whether he sleeps or not.
It's a small space with lots of people. I do what I can to preserve my mental health!