The rule with cartridge bearings is to use them until they wear out, then replace. No way you can say "replace every year" anyway, because one person might have 500 miles in a year, and another person might have 5000 miles in the same time. Add to that all the other variables (riding style, environmental conditions, etc, etc) and it becomes even more meaningless to try and put a set time frame, or even a mileage interval, on replacement.