The last I looked that was an obvious concern, regarding the life. I think overall it was unfounded and most was based on speculation but when I was reading, there was no real conclusive evidence of life shortening, and if it was, it was miniscule.
The manufacturer will tell you you get 2 years life. A hobbyist(like myself) who's concerned with spectrum shifting will suggest to replace every 12-18mo. I would still er on the side of caution and replace every 12 mo., especially if you are overdriving them.
Me personally use Workhorse ballasts. Theres a stigma regarding there instant start, as being too hard and will shorten bulb life. But as it turned out the number proposed was a key word here "potential" to decrease up to 4-5months short. But if your swapping them out every 18 months at least this point will be moot. To add, Ive been using the same ballasts for the past 5 years
-Justin