I regreased my at 25.000km, replaced them at 50.000 and regreased them again at 70.000km. All because slow riding in a straight line became difficult. The bottom one becomes rusty even after applying a lot of grease. I think it is the combination of heating up every time (due to the oil) and a bad seal from the bottom end.

I actually regreased them yesterday, without taking out the wheel and forks, only the steering bar and upper tripple clamp need to be removed. Its a 3/4h job in that way.