I don't want to buy "throw them away when they break" type fixtures. That seems like a huge waste.
If the fixture has a 50,000 MTBF, and I have 50 fixtures, I'll expect 1 to fail every 1,000 hours of use. 1,000 hours divided by 8 hours per day is 125. So, I'd expect to have 2-3 lamps to fail...