Sort of a weird question jake_89.... do you mean how fast can a pair of bettas spawn again after the first spawn?
I guess, it depends on the health of the bettas after the first spawn. Its quite hard on them even if they are not sucessful. I think I read that a 3 month period is suggested between spawns, but I believe it can be more or less.
Betta spawns make anywhere between 12 and 400 eggs. I had over 340 betta fry my fist spawn (still have over 220 left)
Its at least a 6 month commitment if you decide to spawn betta fry. I spend 5 to 8 hours a week cleaning, feeding and generally being daddy to my 5 month old betta fry. Its 12:23 AM and I just finished my water changes for the week.
http://www.bettatalk.com is a good place to research.
Read my thread in this forum called 'Rigel & Ruby - Round Two' to see what I have been doing with my fry!
http://www.myfishtank.net/forum/showthread.php?s=&threadid=11888