Don, like Rex said we already discussed this a lot here.
I even did a Poll a while back that I can't find now, sorry.
The consensus was about half use their bulbs till they die
which can often go over 2 years, while the other half replace
at around 12-15 months when the bulbs are noticeably dimmer.
Personally, I think it matter how much overhead watts you have.
for example, say you have 3wpg, but after a year your bulbs
lose 33% of their brightness, so now you really have 2wpg and
your plants may suffer. That's why I'm an advocate of getting
4wpg to leave more of a wpg buffer as my bulbs dim over time.