Speaking at the press conference in which new companion Jenna-Louise Coleman was unveiled (after escaping a bag by riding on the back of a cat) as Karen Gillan’s replacement, The Grand Moff said:
“Honestly we don’t know [how long he'll stay]. We take it a year at a time and that’s the only sensible way you can go with it. It takes a huge amount of time, filming a series of Doctor Who and towards the end of that, you have to consider whether you’re going to push on to another one.”
Of course, Steven Moffat doesn’t want his star to leave anymore than we do. The Eleventh Doctor has been something of a revelation, especially coming on the back of the very popular Tenth Doctor. Not only that, but casting a new Doctor would mean a lot more PR work – interviews and appearances – for the Moffat which can detract from the main task of making episodes.
“We’ll keep him as long as we can, that’s the absolute truth.”