I had a 1988 Fourtrax 300 2WD that I sold last year. I rode it at least two or three times a week, anywhere from 5 miles to 25 miles at a time probably, depending on how much time I had. I have no idea how many actual miles were on it, though. I think I got it from my grandfather in 1995 or so, and I have no idea how much he rode it. For some reason, Honda decided not to put the digital displays on the '88 models...go figure lol.
Anyway, if I had to guess, I'd say it was somewhere in the 20,000 mile range. Oh, and it ran perfect when I sold it last year, completely original motor and everything.