How long is a piece of string?
Usually bulbs are given a life span in hours rather than miles or months as different people use them for different lengths of time.
The majority of bulb failures are caused by incorrectly fitting them, e.g not wearing latex gloves, the slightest bit of grease from your hands will heat up and create a hot spot on the glass causing premature failure.
Another factor is plain luck, I've had bulbs in my living room last 3 weeks, and others last 3 years. I guess the little filiment is asked a lot of, it gets so hot and is asked to cool pretty quickly, in a car a quick trip to the shops getting it hot and cold repeatedly in quick succession can only reduce the life span.