Wow, just about every opinion possible on this one.
Two points here.
One is that HOW the machine has been treated is vitally important. Along with the basic history of that sled, some machines have a very long life expectancy if you aren't unreasonably hard on a machine and take care of it, with a bit of luck you can put lots of miles on it with just routine maint. and changing out parts which are worn out or waring out. So learn the history of the unit you drive and see what the average is doing and then combine that with how YOU drove it and that should get you the information you are wondering about.
The other thing to remember is, that beginning the day you buy your new sled, you should be chunking $1000 per year in a special savings bank to get ready for the next sled. If you do that, it won't be such a jolt to the system when you buy the next one.
I know it is hard thing to do, just like buying trucks...once the sled is paid for, you simply keep paying yourself by putting the funds in the bank, $50 per week or whatever you can afford then you won't need to whack the wife, lie to her or give up a pool, because the money is there and you are ready with the funds and she won't kill you because you have to kill the monthly budget with your expensive sled.
I do agree that the 4 stroke sled is going to change how we look at the life of a sled...and as others above on here can testify, the rebuild of 2 strokes are all over the boards and you can always get stuck with a machine that has flaky piston rings, or weak crank bearings by design and then the life is seriously shortened.
Take care of a sled that was well built and designed and put together correctly at the factory and you can drive them for 20,000 miles and that includes a few two strokes that were well taken care of. Oil has come a long way to help this out too.
And dats all I got to say on this one....