well, if you just look at the graphic work of a skin, it would take probably anywhere from 1-10 hours. Depending on the detail and complication of the work.
Coding is another story, takes a couple hours to make everything fit perfect, and then there's random updates that you always go through before its final.
To make a skin, basically all you need are three things. A header (buttons and banner) Postbit (what each persons post will go into) and a footer (the look of the bottom.
You also need a color scheme. Option things would be headers on forums, categories ect.
If you wanted to design a skin, you would take a screen shot of the forum you want to skin, of the home page. And a screen shot of the post bit, and then put your graphic work on top of it.