How often should wheel bearings be repacked?

Mileage, Non-use vs Use, Environment, and Warranty are all factors to consider when deciding the frequency of any maintenance. I agree that every year seems too often and I usually do an every other year routine. If you have any kind of warranty then regardless of how you feel you have to follow their requirements.
I also use a infrared thermometer to check heat from the wheels and hubs during trips. If the temperature starts to climb too much I start to look for answers.
Heat is the killer and the first indicator of problems.