Asked in the magazine how long the 50th special is (it's been rumoured to be 60 minutes, but Moffat has denied this), Moffat said:
“People keep asking how long the Anniversary special is, and it’s a fair question. I don’t actually know, because the answer is, it’s as long as it needs to be.The script is quite long though. I’m looking at it now. Ooh, that’s quite long.”So while Moffat is remaining tight-lipped on the length of the special, the filming length does indicate (taking 3D into consideration) that the special is most likely shorter than 90 minutes, but quite possibly longer than 60.
[Source: Doctor Who TV]