I was considering doing the same thing for a month or two and seeing how it works out. My last game got me $1200 up-front sponsorship plus I get +$1,000 for the first million views and +$2,000 for the second million views... and I made the game in 5 hours (it is due for release any day soon!).
I guess it all depends on how lucky you are with if your game is a hit or not really, you could spend 20 hours on a flash game only to find sponsors are sick of those games and only offer you $500 for it, or you could pop out something simple which sponsors see a lot of potential in and offer you thousands for!
Considering I am a totally WANK flash developer and I expect nothing less than $1,000 for my games these days (and I NEVER spend more than 10 hours on a game) I think that full-time flash dvelopment could be a massively profitable job!