Mustrum_Ridcully
Legend
In "Day of the Doctor" he said he was 1,200, but he also said that he lost track and that he might well be lying. Indeed, he said he was so old he forgot whether he was lying about how old he was.
Obviously, we don't know how long there was between "Day" and "Time". In "Time", there's a stated 300-year gap between Clara being sent away the first time and the TARDIS returning. There's then an unknown gap between her being sent away the second time and Tasha Lem returning for her.
The pre-release spoilers said the episode spanned 900 years, which feels about right given the aging of the Doctor in the episode, but it's hardly canonical.
So... my guess would be about 2,000 years. That's a nice round number.
Oh, and also...
On my second watch-through of the episode, I spotted two things:
The prophecy that the Silence were trying to thwart was: "On the fields of Trenzalore, at the fall of the eleventh, when no living creature can speak falsely or fail to answer, a Question will be asked, a question that must never, ever be answered." (Emphasis mine.) So, that one didn't quite link up.
The second thing I noted was a more direct contradiction. Near the end, when the boy who isn't Barnable comes to get the Doctor, he says "don't worry; I have a plan." He then tells Clara that he doesn't have a plan, but that it makes people feel better if they think he does. The only problem is that he's standing right next to the crack in the universe, and so should be affected by the Truth Field, and of course those things can't both be true - he can't both have a plan and not have a plan.
But those are both really just nitpicks. I actually found that I enjoyed the episode a whole lot more on a second viewing, even though I watched it again immediately after watching the (IMO superior) "Day of the Doctor".
I don'T remember when he said this to Clara- he wasn't in the TARDIS or anything, was he?
I figure the Dcotor, after a few centuries, can lie even in a truth field. The Doctor Lies, right?