I think it also depends upon how often you use the ball. If you keep it clean, take it into your pro shop regularly for proper maintenance (de-oiling, slight sanding back to OOB finish, things like that), it should last a good long time. If the lanes you bowl on are flooded most of the time, then it probably will last a year or two. If it's more like you use it one game a week max. then it should last a couple of years I would think.
I live in a dry climate, in Albuquerque, plus at 5,000 ft in elevation and here, things just don't die very fast. I used a couple of hammers for four years, without proper maintenance before I really noticed that they were dying. I learned my lesson there...maintain you equipment!
Chuck