Steven Moffat recently approached upon this subject during a section in DWM, but we are no closer to finding out the true length. We were originally hinted at the 50th being 90 minutes long, by Moffat himself (although sources on this vary widely, and it was a long time ago), where as the BBC's own official banner claimed 60 minutes.

In the latest issue of DWM he says: “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."

He adds: “The script is quite long though. I’m looking at it now. Ooh, that’s quite long.”

Elaborating on how long average scripts are: “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)."

It is hopeful that we should be able to get an estimate in either the following DWM or the one after, as the 50th Anniversary has now completed filming and entered post-production.

[Source: Doctor Who TV]