The "lumen maintenance" of T5's and T8's is about 90% over their lifetime.
Some of my research links are dated, but if anything the data will have improved over the years...
My bulbs are at least four years old, they were two year old when this pic was taken:
The N. macrophylla that just won POTM, is grown under 2 yr old T8's
But, like everything else in this hobby.... ymmv :-)
Bulb life is highly dependent on ballast design and operational conditions.