Fluorescent lamps fail in a bell curve. This means that at the top of the curve your fluorescent lamps will have reached their average rated life. Typically you would expect a large percentage of failures to occur in the range of 80-120% of rated life.
Fluorescent lamp life ratings are determined by the manufacturers on a run schedule of three hours per start. Lamps that are operated in longer cycles, 12 hours per start, will exceed average life ratings by 30% or better. This longer life is achieved because less tungsten material on the cathode is burned on a longer run cycle between starts.
Cathodes are found on each end of a fluorescent lamp and are designed to ignite the gas and a fluorescent to that excites the phosphor and ultimately produces light.
A fluorescent lamp that is turned on and operates 24/7 will burn very little cathode material. Failure in lamps that are rarely turned off occurs when the phosphor coatings deteriorate and the percentage of initial lumens in the lamp falls below 70%.
If your facility has lamps that are failing at a high rate for the light output of your fluorescent lamps has declined significantly, then you are a candidate for a group relighting. The benefit of changing all the lamps at the same time include reduced maintenance costs, improved light levels, and improved energy efficiency.