The show will continue to run as long as it's successful, and it will stay successful as long as it's good and people watch it. Some people are worried that means it will end on a low note, but until the show shows sign of running out of steam, I see no reason for wishing it to end. Look at Doctor Who, that's still going strong, mostly because it constantly reinvents itself, which is the recipe to staying on air for a long time, and so far, I think pony has managed to do that splendidly. People have speculated they will end it with the movie, but I find that extremely unlikely, because if the movie is a big hit, then they would want to do more. I therefore predict at least seven seasons, season 6 in 2016, the movie in 2017, followed by season 7.
People are saying Crusaders of the Lost Mark had indications of an ending, but it was made pretty clear that it was simply the beginning of a new chapter for the CMC, like Twilight getting her wings was the beginning of her finding her ground as the Princess of Friendship, and then fill out those duties. Incidentally, I believe most people also thought Twilight becoming a princess would be the end game of the show, but here we are, almost two full seasons later, still going strong, and with a lot of new developments and great pony episodes.