Steven Moffat has dropped some hints about Matt Smith's last story. The last adventure for the 11th Doctor will be at Christmas as far as we know. Here is his comment.
“The special ties up stuff from all corners of Matt’s tenure – there are things I set in motion in Matt’s very first episode that I’m paying off now… It’s been a long game, but most of the questions that people ask will be answered… One of the horrors of regeneration is that a certain amount of his persona alters entirely. His appetites and his enthusiasms will change. And that’s sort of what I’m writing about now in Matt’s last episode, the fact that he’s terribly aware that he’s about to be rewritten. And it’s frightening.”Source: io9