Matt Smith's Reign as the Doctor

So if David Tennant's length of stay as the Doctor is going to be the standard for the new series (4 years), Matt Smith should be around until 2014. On the other hand, they might cut it a bit short and have him go out in the 50th Anniversary Special (that post earlier saying he would was an April Fool's joke), though they've never had a regeneration on an anniversary before. One reason it might be longer than four years is if they decide not to get rid of the 12-regeneration limit, but want to keep the show around as long as possible before that. I can't imagine them ever wanting to kill the show, this time permanently. I think they're going to try and find a way of making 507 the new limit.