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How often to replace T8 bulbs?

Hey guys, I run my 6500k T8 bulbs 18 hours per day from February 1st to late May/early June. That's roughly 2160 hours per season and I have 2 seasons on them, so about 4320 hours so far. This year will be the 3rd season on them. They're still bright, but noticeably not as bright as new bulbs are. Should I replace them for optimal growing results?

I've read that fluorescent bulbs degrade not only in total light output, but spectrum can also change a bit as time goes on and they wear out.



i wonder the same thing about my cfl's... but every year they seem to do the job just fine... maybe just because they are just doing seedlings to 6 month old plants but they seem to be fine after 6+ years of 6 - 8 months a year 24/7... personally if you are trying to fruit with them i think it would matter, but if its just starting seedlings to go outside i dont think it does...
BUT, im no expert on lights... just saying with thousands upon thousands of hours on the same cfl's i have seen no difference in plants even with noticable difference in the lights output/appearance.. well appearance...to the naked eye they look faded but they dont seem to output less... although i do have 20 of them in less than a 2x2' space so the concentration may be whats keeping the plants going strong... in the beginning i actually have to unscrew half of them to stop darkening and curling
juanitos said:
read the bulb specs. probably 10,000 hours
Thanks guys.

I'll check the rating on the site. They are Sylvania 6500k. They're fairly cheap which is why I was considering replacing them. $35 for a box of 12 at Menards.

But I don't have a use for the old ones and hate to throw them away if they still work