I always plan or replacing them around 4-5000 miles. Grease them every week or so and they will last. This is not only an A/C problem.
I have replaced them on my dads skidoo grand touring, and both yamahas in my shop. A buddy had one go on his mxz around 6,000 miles - in my opinion they are a wear item, just like pistons, rings, skid bearings, hyfax, etc.
I mean, come on, they are located in a very in-hospitable place, if you had as much water, ice and abuse as those poor bearings did, you would fail too.