I bid each job on a set price which is based on $50 per hour.
Typically a single figure, depending on the size and the amount of detail, takes between one and two hours.
You can add about an hour or so for a simple background. Each detailed object adds about another hour.
Of course, the size of the image is important. An ongoing cartoon strip is negotiated for a set cost per strip and the amount of time will vary.
If you have an idea of something you want to do, just shoot me an email and we can talk about it.