I agree with what Trystan said. I don't do painting for a living but here are the impressions I've got from hearing from those that do. There seem to be two routes to go. The first is to paint whole armies to table top standard. You're spending minimal time per figure and output a high volume of work, so an individual piece wouldn't be getting 9's and 10's in the gallery but the paint quality is enough that it would look good for gaming. Your clients are gamers who don't have the time, ability or desire to paint their stuff themselves but still want nice looking figures to game with. The second route is to focus on display painting. Here you're spending a lot of time on a single figure and painting it to a very high standard (9's and 10's in the gallery). Your clients are collectors and companies (doing their box art for example).
There seems to be a larger market for the first route. It's a lot easier to convince someone to pay $1000+ for a whole army than that same amount for a single figure. As Trystan said, I think people who do display figures tend to often supplement their income by running classes, making dvds, books, etc. In both cases there are people who do this as a full time job. It's tough. You need to be very efficient. In this case time really is money. The quicker you can paint to the desired standard, the more jobs you can take and the more money you can make. My impression is a lot of people get into army painting for a bit but end up realizing they can't make it work. But there are some who seem to have figured it out and stick with it. Of course there are plenty of people who do this on the side and use the funds to help supplement their hobby.
If you're looking into this as a job, I'd suggest start with it on the side. See how it goes, how much time you're committing to each project and what sort of rate you're able to get. It will also help you build a reputation. Then you can decide if it's worth jumping into fully.
