I'm relatively new to digital art, although I am a self taught traditional artist. So far, I've been working in sketchbook pro for about 4 months and just downloaded Inkscape last night (dabbled in it for about an hour).
I'm exploring the idea of freelance work to fund a small indie studio and I've heard that one of the ways to monetize is by charging hourly. So how long is reasonable for character concepts, environments concepts, sprites (2D assets), and eventually 3D assets. I understand that everyone works at their own pace but, no one will higher you if you take too long to get things done, making your prices redicilous.
In the end you still analyze how much the whole project will cost. Hourly rate comes in handy when requests with changes start to flow in from customers' side.
So if lets say you have a task to do job X just ask yourself what you consider a reasonable value for it. Then just divide that value by hourly rate and provide an average number of hours required for the project. But when your customer starts asking for more changes just keep adding the hours at your base rate.