Pricing.A lot of new artists don't know how to go about pricing their work. Or even selling it for that matter. I thinking knowing somebody in the industry is the easiest way to get started. Of course, that may be out of the question, in which case, I would reach out to another artist to get a contact or two that you can show your portfolio to.
Selling your designs can be hard in the beginning. I started out selling stuff for $50. Once that grew it went to $100, then $200, and so on. When you build up your clientele to where you can pick and choose work, then you will be able to charge a little more. It's really important to keep in touch with the people you work with and build relationships. Don't annoy them though!
Length of time per design.Really depends on what style you're trying to get and who you're working for. If its for a pop band that is reaching 14 year old girls, then I'd say don't spend too much time on it. Visualize your work before you start trying to get too far. If the job is for something more mature, you might spend a few more hours on it. I try to work as fast as possible so I can get them on their way and happy with what I've done.
But overall, simple designs - about an hour. More detailed, up to 8 hours. My work usually takes me about 2 hours per design from concept to finished product. Of course, that can change majorly if I'm having an off day!
Finally...Don't be afraid to ask what you think you are worth. You want to do a good job, of course, and your pay should reflect that. Don't get pushed around!