I usually go through a pair and a half a season. I buy a pair before the season starts, use them as long as possible, then buy a new pair for the spring, those then get used as rail jam/urban skis the next season. But, I thrash skis pretty hard since I ski east coast park (lot of rails) at least 5 days a week, so I wouldn't really say it's normal.
I'd say "days used" is a better judgment of durability than "seasons used" cause I know people who say they've had park skis last them like 4 years, but only ski on them under 10 times a season, so they really only lasted 40 days.