If you continue to adjust for additional mass and any other requirements to continue adding lean mass - there shouldn't be any diminishing returns until myostatin becomes a problem.
Too many guys stay on, build up, and don't increase their intake. Eventually calories in vs calories out becomes a net zero equation, and maintenance happens instead of continued growth.
Sure, there are other factors - but I know plenty of folks that run cycles that long, and they continue to grow. (they do not PCT - so obviously not recommended for those that do) Just have to keep in mind that there comes a point where "noob gains" stops, and it does get harder to add mass relatively quickly.
I guess I'm saying the same thing as Tron, but in a long-winded manner, haha.