I've heard 6 months and 12 months as well. I've always been able to visibly see my
MH bulbs were dimmer/color shifted well before 12 months, so i usualy swap them at 8 or 9. They could probably go a year, but they could also probably stand to be replaced at 6 months....
As for
T5s, I just started using them when i got my new canopy, so no real experience there, but I hear it's about the same replacement cycle.
