While there are typical good nights and times, it often varies from model to model. I've had some of my best shifts on days that other models were really struggling, and vice versa. I could tell you my no-fail, will definitely be raking it in time slot, and you could work that shift and never make a dime.
That being said, I would check out the site you're camming on for two things: the times that the most models are on, and the times that the least models are on. Typically, when more models are on, there's more traffic, which is why so many models are staying on. Some people also like to cam when the least amount of models are on, because it is easier to be noticed in a smaller crowd.
You didn't mention which site you're on, so it's very hard to give you more concrete tips, because each site is a little different.