For graphic design stuff in general, check out the book "The Graphic Artist's Guild Handook of Ethical Pricing Guidelines". It will not only tell you typical hourly rates, but typical total charges for different jobs- not guitars per se, but it's mighty similar to other decorative arts, illustration, airbrush, auto custom, etc.
Generally jobs like this involve two seperate charges- one for the mechanical craft of executing the design on the object- i.e. painting it on. The other is for the actual planning of the design- if it's something simple you might give them this part for free ("I want a big 01 like the General Lee") but if they want something complicated, and need to have their hand held as you lead them through the design process, giving them options ("I want something hip that I can also use as the logo for my business") then the extra time that will take gets billed seperately.
It's vital to separate the charges this way, because if you don't, and you skip the design process (as fine artists are prone to do- they think their first version will be accepted as 'finished') then you get into the region of hell known as 'design by assasination'- where you do the work, the client rejects it, you do it over, he rejects it, etc etc.
Since we're talking about design process, I'd heartily recommend following something like this:
First meeting: Gather data, find out what they want, they decide if they like you.
Sketches Meeting: Show them 6-20 sketches of possible different concepts. The sketches should not be refined at all, just indicators of different ideas. The client's job is to pick which idea.
Drafts Meeting: Show them a worked-up but not final version of the design. Now is the time to do any fine-tuning, make color choices, etc etc. Often this will mean showing color test swatches, etc. Go over each detail with the client and get their decision/agreement whcih way it should be.
Final Delivery: you deliver the finished work, and they love it. Better yet, they love you: "It's so easy to work with you, I didn't even know what I wanted and you helped me get it." And that's where the word of mouth starts spreading. Your customers are not jsut satisfied, but ecstatic, and they tell all their friends, and you get more work.
If you follow the above process, you'll do well. Note that the process not only helps you do good work that makes the client happy, it also protects you. If the client comes back at the last minute and changes their mind, you say "OK, that will cost extra" and you bill them extra for any work you have to do over as a result. This is a VITAL factor in ensuring you get paid fairly for your work, and a key point in your contract..
BTW, the Graphic Artists' Guild Hbk mentioned above has not only pricing, but sample contracts you can use, or adapt the wording to your own. When the customer is ordering standardized stuff, a simple order form will suffice ("one Mary Kaye strat body in alder please") but when it comes to doing design work you really need a proper contract- and your client will understand that it protects them as well- and makes you look good.
(FWIW, I've worked as a professional designer of all kinds for nearly twenty years, and I teach design at the Art Institute of Seattle, so I'm not just making this stuff up. I've used this process countless times with success, and so have my students, on everything from business cars to custom motorcycles. If I can help with specific details on the business of design, let me know)