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

Based on what I am reading here, I am going with rough roads. Not only do we tow on rocky forest roads, but we tow on California freeways, in very poor repair. Potholes and cracked pavement and mismatched expansion joints. "Shock loading," indeed.

Since there is no way to avoid this (without moving out of state!), I will just have to be vigilant and proactive.