I just wanted to second lions1995 comment and add our own recent experiences.
Over the years (since the early 90s) our BC stays have covered the following months: January, February, March, July and August. The SAB hours vary significantly based on time of year, crowd levels and weather. For us, that's always worked out perfectly - we've never been in the situation where we wanted to swim at night, but SAB was closed. (Although there have been times that we would have liked to use SAB in the early morning and it wasn't open yet, so we just swam in the quiet pool instead.)
Here are some examples from our recent stays in 2010 and 2011:
March, 2010: In early March with cooler temps, SAB was open until 10pm every night of our stay. Our older boys swam, but it was too cold for the adults!
July 2010: SAB was open until 11pm every night of our stay. The weather was nice and warm and we were happily swimming until the 11pm close after a day at the parks!
February 2011: In early February this year, it was freezing! SAB closed at 8pm, which was fine because it was too cold for even our boys to swim at night. They did want to hop in the hot tub one night, so they used the quiet pool hot tub.
August 2011: This was late August, value season. SAB closed at 10pm when we were there. We spent enough time swimming during the day because it was so hot. I was surprised that SAB was only open until 10pm (instead of 11pm) during that time, but we were guessing it was because crowds were lower.
Again, these are only examples and the hours can vary significantly. We always found it's best to ask the lifeguards for operating hours, instead of the front desk. The lifeguards have always been aware of last-minute changes to the hours. Hope that helps!