@camoceltic Big clue to find ones meant to be serviced: Look for ones with visible screws towards the bottom of the ear cups and a removable audio cord.

And eh, it varies.  There's a lot of conditions that affect the life of electronics and since people wear them in ALL KINDS of environments headphones can have variable lifespans.  But it definitely helps multiply that lifespan several times over when the headphones are serviceable.