I've not had the same bulb for 15 years. I've changed wattages and voltages a few times. Some of the bulbs have been replaced with wider angled beams but, in the 15 years of using the MR11, I've had two failures that I can think of. One was from the mirroring on the reflector coming off and coating the inside of the lense...still useable but rather diffuse and the other was from the bulb burning out which was last year. I don't keep track of the changes but from what I can recall most of my bulbs would have been in use for around 5 years.

I use multiple lamps when I ride also, so I've got more then one bulb in play at any given time.