How many miles would you expect an alloy frame to last, given normal road use and average weight rider?

Alloy frames are a new thing to me. I've got three alloy bikes of my own and my sons have three more. Earlier bikes we have owned have all been steel framed, and some proved very durable and tolerant of abuse over more than twenty years before I threw them out. I just gave away a fifteen year old mountain bike (?must be about that?) which had suffered a slight bend in the back end when my eldest son got to that kamikazi stage of hurling himself down precipitous, rocky slopes. It still worked fine, except that the alloy stem and seat post were utterly siezed into the steel frame. An old guy I know is toddling about on it now and I can confidently say it will see him out, since he's 82 years old. He had this awful ancient single speed ladies bike that he could hardly pedal into the wind anymore. He thinks the 21 speed Marin is a revelation.