Robert Kirkman said he had a roughly 300-issue run planned for TWD comics. This works out to 25 years!
Thing is, I hope they will continue to go from one overrun place to the next, at least for now. This seems to be the only realistic way of gettign the characters to explore the post-apocalypse world... However I'm sure eventually, once humanity as a whole becomes used to life-with-zombies, we'll have several established communities with our main characters braving the wilderness to go between them. Sorta like the world of Revolution, with less living people.