I'm an electrician and have to schedule light changes in our facilities. Most manufacturers say that lumen outputs start to deteriorate @ 4,000 hours of use and I use that as my base.

I don't know how many hours before it would affect plant growth, since I'm new to that part.

If you have them on 12 hours a day your 4,000 hours would be up in a little over 300 day's.

I would replace them.

frances_in_nj, I am by no means an expert but did a lot of research on the topic a while ago and have a little data with a light meter. While all lamps are different take this information as a broad generalization on T8 lamps and other fluorescents.

Typical fluorescent lamps lose between 10-15% of their output during their lifespan, CFL's can be up to 25%.

The lifespan of T8's can range from 8,000-30,000 hrs with most falling in the 20-25,000 zone. With new technology some companies are claiming over 35,000 hours, of course at a cost. The most detrimental thing for fluorescent lamps is turning them off and on. With under 5 cycles (off and on) per day lamp life can often be doubled.

The most noticeable light loss is at the ends as they go black.

Above was all based on research.

I did have the opportunity to test my lamps when they were new 3 yrs ago and after approximately 4500 hours of use and around 300+ cycles both my T5 and T8 lamps lost about 1-3% light output. The T8's were not new when I tested them initially and I have no idea how many more hours are on them as I got them used.

Now should you replace your lamps or not? Well if we go with the data I researched and you ran your lights 15hrs a day for 240 days a year for 5 years you would have 18,000 hours on them. If they are rated for 24,000, which is a normal figure, you are at 75% of the bulbs life. BUT with low cycles you possibly could get 48,000 hours out of them and only be at 37.5% life. So you have lost somewhere from 6-12% of the light output of your bulbs, give or take. In the end it is your call, if it was me and my plants were doing well I probably would leave them in and save that money for more seeds......

SCG

Thanks for these responses! Since you both offer different opinions, I'll have to give this one some thought.

I do appreciate your help!