When we were there last summer for the July 4th week, SAB closed at 10pm. However, I do remember the previous summer that it closed earlier than that.
I guess the hours are subject to change, (as are all things Disney). I don't know how they determine how late to keep the pool open.
You could always call the YC or BC manager closer to when you are arriving, and ask them if the hours have been set yet. I'll post when we get back from our July trip with the hours it was open this year.
Thanks for posting Cindala. I know you really like the YC. I want to give the BC a try but I just am a little worried about how busy SAB will be this July. I never thought of the pool closing early. We always stay at the Polynesian and we love to go swimming at night, it's fun to watch the fireworks while swimming If SAB is open till 10 I could hang with that. I just wouldn't be interested if it closed around 7:00