well, I just looked in the sport 1100i workshopmanual, there they say replace each 15000km with sporty use or each 20.000 km with normal use. I know they live longer than that. But anyway it is good to take them of each 10.000km, have them in your hand and feel if they move without play. And while you have them there, it's the perfect moment to grease them.
And I'm no saint either, I once had one with melted bearing cages. But that was the sort that didn't have grease nipples.