How much money depends on how much you need to sustain your lifestyle. You need to be able to make a consistent profit of your living expenses plus about 1/3 of that profit for taxes. If, as a pro, you can write some things off of taxes legitimately, you probably don't want to factor that in until those expenses are plainly legitimate and substantial.
So, first, you need a bankroll that will easily let you play at the necessary levels. A live bankroll might be different than the needed online bankroll, as, online, you can multitable. Online, also, you can adjust the hours played as necessary without factoring in drive time, gas, etc. So, if you need to make $1k (+350 for taxes) a week, if you played 40 hours a week, you'd need to make about $34/hr, so you'd have to play at whetever level you'd need to make that. Maybe you'd need $50k to start as a live pro but only $20K for online, as you could multi table for longer at lower levels with similar possible results. You might also be better able to win at the lower levels, and better able to psychologically withstand beats.
Then, of course, you need to factor in variance. Those days, weeks, months where you are not making a profit, and eating into your bankroll. You'd need a plan of action -- grind out more hours, perhaps, at lower (beatable) levels. Or put a hefty slush fund aside to help you ride out those periods.
Personally, I would think that making poker my primary source of income, my business, would drive out a lot of the fun of playing. Knowing that I absolutely have to make $XXXX a month or my bills wouldn't get paid... yikes. I doubt I would like the pressure, and suspect it would make me tiltier. I suspect that's why so many pros welcome affiliations, sponsor deals, book deals, TV, other ways to bring in income without having it all ride on the turn of the cards.
Obviously, I've thought about it. It's a no for me. Beside, I don't have the bankroll... LOL