I agree that every market is different. Think about who your target audience is, (i.e. stay-at-home moms, working professionals) and that will help guide you. I offer 3 different boot camp classes M-F, 5:30 am (2 days/week), 9:15 am (5 days/week) and 5:45 pm (4 days/week). I used to offer a 1pm class (3 days/week) as well, but the most popular times are the first 3 I mentioned above.
I think mornings before work, around lunch time for the lunch rush and evenings after work.