They can last forever, it seems

Even if the rings or valve seats went, good barrels and pistons are dirt cheap (£25 the pair) and heads not much more, since most broken SV motors fail from the bottom, not the top. You can get a whole engine with a good top but bad bottom for practically nothing.
But then, they can go relatively young, depends on a lot of things- usage, servicing, and luck I'd say.