Haha Heritier isn't a legend though, he's more of a Maison Mere legend because of his long spell there but as an actual solo artist legend he has a long long way to go.
How much time should pass before an album can be assessed as a classic? In my opinion, it's probably 2-3 years that an album still plays at parties, clubs, and certain songs remaining a staple at concerts. With albums like of Qui Est Derrière Toi and Arsenal de Belles Mélodies, I already saw people considering them like legends as early as 2012. Around that time, they've followed in the footsteps of their mentors by performing at important halls like Olympia.