The 3D special - airing on November 23 - had originally been reported to be 90 minutes long, but recent rumours have suggested a one-hour run time.
The executive producer suggested that the length of the episode has yet to be decided.
When asked by Doctor Who Magazine, 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."
The episode is expected to get a limited worldwide cinema release.
Who veteran Nick Hurran directed the episode, which wrapped shooting earlier this month.
The special stars David Tennant, Billie Piper and John Hurt alongside Matt Smith and Jenna-Louise Coleman.
Watch Smith and Tennant discuss the 50th anniversary special below: