You could go by a lot of different things:
The good times for you to cam are when you personally can get online in a good, relaxed, happy, and horny mood. After all, if you're not in the mood to cam, you're just going to drive your fans away.
If you don't have a high camscore, it's suggested to avoid the major time-blocks because there's too much competition and you won't get anyone in there. Thus, another thing to do is to check out the site in guest mode and see what the last camscore on the first page is. If yours is lower, you might not want to try it.
Or, you could figure out a schedule that you're capable of sticking to and go by that. Getting on at a consistent time for each day of the week is a good way to make sure that the people who liked you last Wednesday can find you this Wednesday.