I played mostly micro-limit for about 3 or 4 months for about 20 hours a week after getting a few months of experience under my belt. I'd suggest starting at 1 / 300th whatever you put in as your BB. So if you toss in $100 then you're looking for a BB around $0.25. Once you have 300 times the next level up in bankroll then go ahead and bump up to that level and move up that way.
I probably wouldn't put in more than $100 if I wasn't sure whether I could hack it or not but that depends on what type of money you've already got sitting around - to some people $10G is pocket change whereas to some people $100 is a ton of money. Whatever you can comfortably lose and say, "oh well, it was worth a shot" would be what I'd start with as my bankroll.
I'd probably start with a few days at their absolute lowest level possible just to get used to the software and the level of play and then bump up to 1 / 300th your bankroll at that point. If you're going to be putting in 40 hour weeks you should progress pretty quickly. The lower levels you can pull in about 3BB an hour with textbook play so that's about 120BB a week...you should double your money in 2 1/2 weeks on average at the lower levels but that earnings per hour will probably drop to between 1 and 2 BB per hour at higher levels (assuming you're still playing better than your average opponent).
I wish I could give you a story about moving up and up in levels but I was doing this more for fun and to see if I could do it than to try and do it as a living - I really like my day job and have no desire to be a professional poker player. But I can tell you that the money was pretty steady even though I was playing at micro-limits and the only thing that kept me from moving up a level every 3 weeks or so was the fact that I kept cashing out so much of my money to spend it on other things. Of course I blew most of the profits on playing in live super satellites but it was worth the experience of getting to play in them.