I was just thinking about this actually. I got a pair of $2 boots at a garage sale for my first ones, those lasted through my first season (12 days for me?).
Second pair of boots were my brother's old ones. They were too big, and really crappy. They smelled real bad. Probably 25 days on them (from me, two season from my brother, and he bought them used.)
Then I got another used pair (which probably only had 10 days on them) and they went for about a season and a half. I put maybe 55 days on them before getting the boots I have now. I got them a size too big, and they STANK.
Currently I've been riding Salomon SPK's, which I bought used (with maybe 25 days on them) and I've had them for 3.5 seasons. They probably have 200-225 days on them.
I just picked up some Rossignol B-Squad Sensor 110's on the Steep and Cheap sale yesterday. $240 (MSRP of $700 according to Backcountry). I'm hoping to get at least 4 seasons out of them, but they don't have replaceable toe lugs (but they do have replaceable heels?). I tend to wear out my toes quick, so I'm considering getting some cat tracks for them.
companies say the average life span of a boot should be 120-150 days. for some thats a long time, others that means a new boot every season just about. as long as you take care of them you can should be able to get 200+ easily, and maybe with some new lugs and foot bed hit 250-300. OR you can just wear them for 10+yrs until something breaks. haha