There are a lot of factors to consider. Your skill level, the amount of time it takes you to make a commission, and the materials/resources you need to use up to make them are important.
Are you planning on doing them as a hobby, or do you want to pursue this as a job? Hobbyists usually price their commissions lower, because they aren't relying on them for income. One negative side effect of this is that more serious full-time commission artists can get priced out of the market, since cheaper alternatives are available.
I mostly see people charge a flat price depending on the type of commission (sketch, painting, full body, bust, etc) rather than charging per hour, but it might be interesting to come up with an hourly rate you want to earn and time yourself for each type. Then you have something to base your prices upon.