A good disc will last until you lose it or break it. I like them better the more I throw them and the older they get. I have never noticed a disc getting harder but the old D or Dx plastic does get more brittle with age. The newer candy plastic will last longer than your next girlfriend. Maybe by a factor of ten.
The theory that I still have my putters from years ago is an urban myth. Back in the day I was one of the worst putters to ever play the game. After a bad tournament I was known to take my putters and throw them into the pond. So all my old putters, through no fault of their own (purely operator error) are no longer with us. Too bad, I threw away some great old Magnets.
These days I like really broken in Rattlers or pretty fresh Magnets for putting (one overstable, one straight, depending on the putt).
Depends on skill level and the course you're playing. If the terrain is rocky or lots of pavement around they can get beat up super fast. If you like to crank 'em off trees and buildings, they can get beat up pretty quick too. This really goes without saying, I guess. The old CE is probably THE Most durable i've ever used, with Star and Champion not too far behind. A disc should most likely last you a whole season if you don't lose it first. Yet again that also depends on how many rounds you throw in a given period.
As a general rule, if you can avoid skipping them off of sidewalks and blacktop surfaces your discs will be around much longer. Trees really don't hurt them nearly as bad in my opinion. And by all means Do Not let them get run over by any vehicle!! From my own experience, this is probably the fastest way to age an otherwise youthful disc. Hope that helps. :)
I actually perfer having 1 beat dx roc along with a newer one just to have two flight paths for upshots and such. I also like my beat champion orc for the long low flat striaght shot. I once had a pat pending (cream dream) aviar until someone took it off the wall and power gripped it......they punched their thumb right through it. Old dx plastic will tend to become brittle. Also dont hang your old dx discs on the wall if smokers are around alot. this will leave a nicotine residue on them making them brittle as well.. most of the new plastic is indestructable in my book....unless its -10 degrees out .