DIfferent brands and different bulbs have different effectiveness curves. A general rule of thumb is 6 months or less for replacement. Some bulbs have whole lives 8000 hours long, some have 20,000, so somewhere in the middle of those, you want to replace them.

Sam

What about CFL's? Should these be replaced before burnout as well?

Just like I said above, it is bulb-specific. Most bulbs will have some sort of indicator of lifespan on the packaging. By about the time you hit half of that lifespan(if 20,000 hours, then 10,000 hours of less), you will have lost an appreciable amount of light output.

This goes for all fluorescent bulbs.

Sam

Thanks for the help, GaWd.

No wonder I can't find the right flourescent bulbs in the stores this time of year!!! Walmart and Home Depot were completely out of Plant&Aquarium lights and only 1 cool white left at HD. I was planning to replace 4 of my 8, but now after reading this, I better go find a lot more somewhere. I was guessing that *some* of mine were very old, but I am sure that *all* of them are somewhat old.

Pushing this to the top as it is very timely!

CFLs generally have somewhat shorter overall life than a 48" tube (as little as 6,000 hours on a 3 hour cycle) and the light generally drops off a little more too (up to 25% by the end of life for some bulbs). If you don't have a light meter you can check bulb brightness by eye by comparing next to a new one. The 10%-15% brightness drop in an older bulb is visible. Reflect the lights in a pane of glass if you want the experience to be a little less glaring :)

Of course, you don't have to replace tubes until they fail, just make sure that you have enough light for the plants. I move bulbs away from the plants and use them as regular light bulbs after a few thousand hours.