Personally i never replace any of my bulbs til they actually
go out (that goes for houseplants and (freshwater) aquatic
plants)... but it should also be noted that i don't have
anything real special/sensitive

I have some 40 watt fluorescent U-tubes that I installed twenty years ago and use at least a few hours every day and they still work fine. I did have to replace the ballast though a few days ago. As the old expression goes, if it ain't broke don't fix it.

THanks for the replies guys.

Flourescent tubes light output drops significantly over time, our eyes just can't detect it.

Typical tubes output falls by about 50% after a year or 2 (If I remember correctly), hence we're actually using the same amount of power, but the plants are getting only half the light intended.

My question was to see if anyone could point me to any research that can tell me how often to change the tubes.

I usually change all tubes once a year since these are my plants' only light source.