Every 2 weeks - that's about when it starts to chip terribly. I tend to slather on the top coat, and reapply top coat at the end of the first week...and I try not to slam or scratch things.
But after being inspired by all the pretty nail art I see, it makes me wanna switch to every week...this may happen anyway, since I've started trimming my nails every week. And when I do that, the nail polish has to change (chipping).