Once a year, because my tanks are always filled and I have no room for more unless I return someone. I hate doing that. And I refuse to get more tanks (for the moment) because I don't want the additional maintainenance. It seems to be an annual fall thing for me. I'm stocking a Tang tank and planting it right now. But I PLAN tanks endlessly. I just don't set all of them up. Right now there is a reef and a hap/peacock on paper.
When I was first setting up my tanks it was more frequently, maybe bi weekly. But now that my tanks are up and running and fully stocked I dont' usually buy anyfish unless i'm replacing one that died or setting up a new tank.
We have aquarium club auctions every 3-4 months so I usually sell a couple fish there and end up buying new ones.