As often as you want to, or only once. :-D
If you want to you could pull a frame at a time, I've heard of that being done.
For me, though, it is 2x a year...once in early summer and once in the middle. After August they keep the rest.
A lot depends on how many supers you have. If you have enough then once is sufficient. I don't have quite enough, so have to harvest more often.