According to Moffat, the Anniversary Special will "be as long as it needs to be
. The script is quite long though. I’m looking at it now. Ooh, that’s quite long."
There's then this bizarre Moffat quote:
In theory a 45-minute episode of Doctor Who ought to be about 55 pages. But it’s a fast-paced show, you need something to cut, so let’s say 60 pages. And that, let me tell you, is wrong. The script for The Time of Angels was 70 pages long, and it came in short at 40 minutes, even with added material. The script for The Snowmen has almost the same page count as A Scandal in Belgravia (sorry to mention The Other One) but one is an hour and the other is 90 minutes. Thank God, the BBC is kind to us. Our notion of 45 minutes has veered between 39 minutes and 57 (I think, roughly this is from memory).
I say "bizarre," because scripts are written one page per minute of screentime. If Moffat is getting radically different running times from scripts the same length ("The Snowmen" and "Scandal in Belgravia"), or if he's getting a 40 minute episode out of a 70 pagre script ("The Time of Angels"), then he's doing something really wrong.
I still think, based on shooting dates, that 60 minutes is the likely runtime.