It's best to test the bulbs with a lux meter to determine when they are beginning to fade.
For magnetic ballasts 6 months is about right. For IceCap ballasts it could be anywhere between 1 and 2 years depending on the type of bulb.
In my experience URI SuperActinic lasts on average 13 months and URI Daylights last about 24 months before light output decreases 15%.