6 years and counting for me. Depends what you do with it. Use it everyday it's bound to put stresses on the welds, especially head tube and seat area join. Leave it in a room untouched for 25 years and it should ride as good as the day you got it because I don't think alloy softens with age, not like some steels. So theoritically it should last forever.

- I know some alluminium frames come with lifetime guarantee*if used ONLY for XC.