Approx how many miles do you get on a set of bearings?

I have my bearings checked and greased as per Jayco's recommendation 12 months or 12 thousand miles. It isn't a big thing with me as after a year of trailering I put 12 to 15 thousand on my trailer. The dealer looks at all the important part and inspects brakes and many other things. For a few bucks I get told whats what and what is looking to fail.