Well, Colassal, and Armor has been revealed, the ability to control them has been revealed, only thing now is to see who the beast titan is, and whats in the basement. Once those are revealed, then its over.
I actually hope it ends soon. I don't want it to be milked out.
A lot of anime series I love are usually just over 24 episodes, and are great. I know Attack on Titan will be longer than that, but I don't want it to be so long, that people will just get bored of it, and the animators will get lazy.
I say the manga should be wrapped up in the next 15-20 chapters. In my opinion.