We have 75 buildings in our school system. Most are 277v on the lights and most are T8 2 or 3 tube. It's not uncommon to see ballasts last 7-10 yrs. With that said ... most of our rooms burn all day (no repeated on/off cycles); I suspect that has some effect on the life. Our newer schools have occupancy sensors that shut off the lights regardless of the switch position but they haven't been in long enough to see if it affects the ballasts. The ballasts and bulbs that don't seem to last are the gym fixtures that use 8 compact fluors (4 ballasts) per fixture.
