Any good ways to estimate hourly variance?

[Foreverastudent]

I'm sure a post like this has been made before, but I can't seem to find it. I apologize in advance for repeating a topic that has probably already been covered.

I've been tracking my progress at a local casino for a while, and I'm trying to figure out a good way to find a confidence interval for my hourly rate.

Now, finding mean is easy. I've got no problem with that. What I don't know is how to find a good estimate on my variance for each hour played.

When I play, I record the length of my session and my winnings/losses. For example, I can look at my records for yesterday and see that I made $116 over three hours thirty minutes, but I don't go into more detail than that. Obviously, these sessions vary in length, my shortest being about two hours and my longest being about twelve.

Don't worry about being too technical in your explanation, as I have a pretty strong math background. Any help would be greatly appreciated.