I do a lot of commission work and the reason I do commission work is because it tops up my income and makes my hobby self sufficient, so I paint part-time. This does give me some degree of flexibility in that I can pick up what commissions I want and when I need them as I'm not solely relying on it to pay the bills. I often find that I'm painting other peoples stuff and my own projects just get left on the side line because paying customer demand you spend time on their models first and foremost, which is right.
I paint every single day for 4-5 hours a day (my choice) and there is times where you just want to pitch up in front of the telly and chill out but I find you can't really do that and you have to force yourself to paint. This can lead you to burn out. With that in mind you need to keep your own projects on the side to keep you sane make sure you allow time to chill out.
It sounds cool being a commission painter but the grass isn't really that green. You need to be able to put out a decent job in a reasonable time and often you will be painting to deadlines.
I thoroughly enjoy painting things for other people, I get great satisfaction from my customers reaction when they see their minis all painted. but there are customers who constantly change things, are never happy with it, think because there paying for table top quality they'll get a display quality commission but they are few and far between I find.
I'd advise taking up a couple of small projects up for your local gaming store and getting your toes wet. Its not for everyone and if I was financially stable I wouldn't do any commissions even though I enjoy painting other peoples stuff a lot, I still prefer to paint my own stuff all day long.
Good luck