Commercial brewers passivate quarterly. Homebrewers probably don't need to very often. If you get a metallic taste in your beer then you need to passivate your stainless. If you get rust spots then you also need to passivate.
Citric acid is what I would recommend using. I'm not sure exactly the percentage off-hand (4%?) but soak, drain and then leave to dry overnight (which will create an oxide barrier, then rinse.