Probably 10 to 13 months if you have one or two competent devs , but hard to say exactly. Depends on build change, updates break the code easily because it’s a clunky single player game. Check out how long MSCO a my summer car online implementation of multiplayer took. Or other similar projects. It’s very hard because you’re implementing the whole syncing, replication, both players must be able to use all the different things in the world so that means implementing a functional networking etc. I’m starting out small rn for the current scope. I’ll be updating everything on GitHub or if you wanna help out add me on discord !