It can depend on a lot of factors and isn't purely based on mileage. Age, type of usage (always 2 up?), care etc etc.
A couple of things to note: -
- Pressure washers if not used carefully can dry bearings out.
- Lack of usage can harm bearings as leaving the bike resting for months on end puts all that load onto 1 point of the bearing for a long time, best to leave bike on centre stand and front paddock stand keeping wheels off the ground
- Have the bearings previously been replaced? If so, cheapo parts?
- Winter riding and lack of care, salted seized my sprocket carrier bearing on my FZS600 about 30,000 miles ago.
Not much help but maybe 1 or 2 of these points strikes a chord with you and helps you understand why yours has failed?
Brace