IMO, it's going to depend on how much you fly and where you fly and store your aircraft. Mine for example, have hung on the ceiling for about a year. I will end up taking all of them apart and cleaning them out. You don't want any dirt or dust in them. As gaba stated, you will need to lube the bearings, but more importantly, you need to clean the bearings. This means, every 50 or so flights, take the motor apart and clean the bearings with brake or carb cleaner, then re-oil them and put the motor back together.
Gaba brought up a great point. Even the smallest rock will damage the windings in these motors. So always check for damage and discoloration of the windings as often as you can.
And this goes for ALL brushless motors.