How long should I let her sleep?

I'm a little concerned because it is 11:10am right now and my 15 month old is still sleeping. She is usually up around 7:30 but has known to get up as late as 9. She did go to bed about an hour late last night and was up a lot throughout the night. At one point it took me an hour to get her back down. All this is unusual for her. She was in a great mood yesterday, ate/drank with no issue. No fever or any other indication of illness. I'm all about "never wake a baby" because if they're sleeping, they need it, but sleeping this late has me a little worried. Thoughts?