I have seen a link to a graph somewhere (I think here) showing how the usuable light in a flourescent tube decays rapidly, but I can't find it anywhere. If anyone has it bookmarked I would really appreciate it. Thanks!
This curve is different for each fluorescent lamp, depending on the phosphors, the ballast, the lapm shape, and the operating conditions. A good bulb datasheet should include a lumen maintenance curve. Most fluorescent lamps will lose 10% of their output quite rapidly, and then another 10% more slowly. Some are better and some are worse.
Here is a link that might be useful: Datasheet with lumen maintenance curve
If you run them 24/7 they really need to be replaced at least once a year but I generally wait until one burns out to do it :)
I have been running at least 2 - 2x4 light shelves for over 20 years, and at times 10, so I have grown a lot of different things under them as my plant interests have changed as the years go by :)