I believe the bulb's usefulness is rated in hours, not months or years. It depends on how many hours you operate the lamp. I can't remember for how many hours an average hid lasts. That is to say, how many hours it will perform within an acceptable range of lumen output. The average lifetime of an average hid might be 24,000 hours, for instance, but that's not what we're concerned about here.
I stopped using my last HPS when I noticed that it wasn't performing as well. I used it for three years; however, of course, I didn't use it continually for three years.