I add air about every other week. However, my kids bounce around on it constantly, so that's a factor. What brand did you buy? Stability balls are not equal! I have a Thera ball and it's very sturdy.
RE: How often do you need to add air to your stability ...
Thinking about it, mine has been needing air for a while now. Of course the kids think it's a toy to bounce around on, and play w/ the dogs w/ it...I am really surprised it isn't popped by now.