My personal experience says they'll last until the first deer jumps out in front of you and destroys the bike.
As long as you can avoid that, they'll keep going for as long as you maintain them, or at least are willing to do so. I don't think I've ever run a vehicle until it was past useful life and I've put quite a few miles on several of them. I'm usually tired of it and lured on by something newer, fancier and shinier. What the subsequent owner(s) do may or may not contribute to the vehicles continued longevity.