Terraphon, As others have said, it depends. I probably should have said every two weeks or so, since that is a bit closer to my average, though I have had some that decrease dramatically in performance after about a week. Bear in mind that I am a heavy user.
I used my first atomizer for almost a month and it still worked (and I thought nothing had changed), but when I tried a new atomizer the difference was surprising. The old atomizer had degraded in performance, but gradually, so I hadn't noticed it. I've been swapping out atomizers roughly every 10 days or so since then, and I'm pretty pleased with the results. Seeing as a 901 atomizer costs roughly the price of 2 packs of analogs, and it's keeping me off of a pack and a half a day habit, I think the price is worth it.