Given how long CD have developed the game, I think the game could have a 6-8 year lifespan with story or world expansion.I wonder when the next-gen patch will be released in relation to the DLC. @BlankStar doesn't know this but each of the Witcher 3 DLC chapters was as big as some standalone games.
It will be interesting to see what multiplayer will be like and if they add other non-story campaign modes.