Replacing working lights?

[jmsieglaff]

I've read here and elsewhere about replacing bulbs after a period of time. The lumens drop and our eyes can't tell but the plants can. I hate throwing away good bulbs without having a legit guideline. I've read after two seasons, but is there a better guideline by hours? Perhaps the bulb quality matters too, I have 6400k T5 high output 4' florescent rated to 5000 initial lumens. Last year I also added a 2 bulb T12 (one aquarium/plant bulb and one 6400k daylight bulb) as I had more plants. I appreciate any info, especially any studies related to bulb lumen output vs hours used.