I will say 4-5 years.
There is no reason to have a different umpire's strike zones every single game.
You should not have to adjust to a certain ump's tendencies game in and game out.
Too many of them are awful and inconsistent.
I see nothing wrong with a machine doing the pitch calling in an MLB game.
Less arguing, and you know it's 100% fair and accurate after they master it player by player.
I gotta think it's definitely possible in the near future.