I've seen several types of commissions. The first is base price plus a revision price. For each revision, the revision price will get tacked onto the base price. The second is by pixel. For example, for a 800x600 image can either license for $48.00 or for the buyer to keep for $480. Another is by hour as was already mentioned and the last is by what is being drawn. A set price for b&w, color, a full body, a half-body, backgrounds, no backgrounds, an additional character, etc.

Personally, I like the base with revision price and in some cases the pixel price due to how it's cut and dry. You can basically draw anything without having to work out the pricing for each detail in the drawing. I pass on hourly because sometimes I either don't mark when I start and end or it's tough to monetize when you worked on something for 2 hours, 37 minutes and 25 seconds. Not only that but the buyer has to trust you to charge them correctly for the hours worked on the project.

On a side note, if you are getting a lot of work and don't have time to do all the requests - raise your price. It's a good indicator that there's more demand than supply and in such cases, it's alright to charge more. Also, if someone wants something done fast, charge extra. It's like getting something shipped to you the next day. If they want that service, they should pay for it.

There's a few other things but I think you're sticking to digital-only commissions so I'll leave it there. Hope that helps! : )