We were told by our dealer that they see most WW's replaced around 100k. We're at 175k on one side and 125k on the other, and still going strong. We do 99% of our own maintenance, and we do a lot of "preventative" maintenance as well.
Not to toot my own horn, but our dealer said he has never seen machines with as many hours as ours look as good as ours do. You can replace just about everything on them to keep them running like new, and it's pretty cost effective if you do the work yourself.