Lighting question: How often should they be replaced?

[Ephraim]

i change my white bulbs every 6 months, but you can still get decent performance out of them up to 10 months if you have them fan cooled. I find that i can notice a difference in the intensity of the light at 6 months by eye so thats why i change mine at 6. actinics i change when they burn out, they give off little usable light, if any, for the corals anyways.