Planted Tank VIP
Join Date: Jul 2002
Location: Victoria,British Columbia, Canada
It really depends on a lot of factors. I built a canopy once that wasn't cooled as well as it should have been and I was able to measure a PAR decrease of over 40% within 6 months of installing new bulbs. When I upgraded fans and improved my cooling it took 12 months to see the same decrease in PAR output.
What was really interesting was also how the bulbs themselves burned in. On the cool air intake side of my canopy, the bulbs would age much slower then the hot air exhaust side. This led me to rotate my bulbs every 3 months to get a more even burn in.
Cheaper ballasts (or dimming ballasts) will also age bulbs faster then a higher quality one, so this is also something to consider.
From my own standpoint, I replace my bulbs about every 9-12 months if they are burning from 8-12 hours a day. If I am dimming them I replace them every 6-9 months. I am a bit anal however, and you can most likely stretch out your bulb usage even further.
Its difficult to pin it down an exact time as lots of things affect how long your bulbs will last.
The best way to tell when your bulbs are due? Rent a PAR meter and measure your bulbs when new, then after a few hundred hours of use, then again at 6 months. Watch the changes. Some fish stores in my area rent them, but you may not be so lucky.