Just got some really great ceiling fans for a client's house.
My client asked for the weight of the fan, which is less than 20 lbs. In passing he commented that he had installed a blue plastic ceiling box that could hold 50lbs and asked "That's enough, right?"
Well, there's a bit more to it than that. With ceiling fans you can't really just go by the weight, alone. The vibration adds to the downward force on the box, so the box needs to be sturdier and attached more securely than a box supporting a light fixture. Outlet boxes that are designed for ceiling fan support have special braces that extend to contact or straddle adjacent ceiling joists. A box that is rated for used with a ceiling fan will be labeled as such inside. It can usually support up to 70 lb and can handle the vibration of the fan without the material wearing out with time. You definitely don't want to have your fan take off on you, so checking the ceiling box is crucial.