I've thought about the subject of publication with chapters and with the anime for quite a bit, and from what I can see, it looks like the anime will end up with 3 seasons, and the manga will end up with 100 chapters. It should finish being written somewhere from 2017 to possibly up to February 2018.
This was based off of statistics based off of when the first chapter was released, September 2009, comparing it to Chapter 50, which was published in September 2013. Comparing to that, the series should end in the 2017 arena, but if the chapters continue to strictly be released monthly, then that's what would calaculate to february 2018 if there are meant to be 100 Chapters in the end.
Why am I saying 100 Chapters? Hajime Isayama's editor stated that there would end up being 25 volumes, and volume 12 is the one that ends with Chapter 50. That would mean 24 volumes, but I just think they might plan to end it with a solid 100 heheh. I don't know of any completely solid sources on this 25 volume thing actually, but from what people have said, it sounds at least like there will be more than 20, trying to flesh out the characters and plot as much as it can instead of having the original 20-volume plan. If anybody finds a good source for it, it would be great if you posted it here!
And about the anime having 3 seasons, easy: Season 1 ended in Volume 8's events, and not only is that close to a third of 25 (and there's a synchronicity with the episodes :P) volumes, but also note the fact that it covered about 33 chapters. 33 * 3 is once again close to 100 Chapters.
What do you guys think? *Bows*