I would say probably one and a half to two years on the fast side depending on how fast they want to complete the story versus how much they want to expand the world.(of course not accounting for any IRL troubles that could delay updates) It looks like they have plans to expand the world adding more for the npc department and a bunch of similar stuff. They give an update roughly every 3 months with the latest being 3.6 which released a couple weeks ago.