If the high speed release schedule continues at this pace, they should be done within a year an a half roughly.
They WILL get new plastic kits - I could see them getting a new plastic character in the form of a Sanguinary Priest. There was a bit of fluff pointing to marines that have gone mad from black rage ect still living in some tower (?) - perhaps they could turn those into some kind of vampiric half-naked marine unit used as some form of terror attack led by a chaplain/priest when times are really desperate, seems like something they'd shoehorn in to me. I'm sure they'd get some form of new vehicle or centrepiece kit - like a new flier, since their current one isnt unique to them any longer.
Kind of hard to think what they will add to marine armies that were updated in 5th ed really, but we know they WILL get NEW units - wheather people want that or not.