Reddit reviews Doctor Who: Tales of Trenzalore: The Eleventh Doctor's Last Stand

Not in the episode, but they released books or novels about the battle of Trenzelore that state he was there for over 800 years.

Edit: Here, its actually 900 years.

>As it had been foretold, the armies of the Universe gathered at Trenzalore. Only one thing stood between the planet and destruction - the Doctor. For nine hundred years, he defended the planet and the tiny town of Christmas, against the forces that would destroy it.

It's called Tales from Trenzalore. I enjoyed it but I would definitely like more, in any format.

There's a book, Tales of Trenzalore.

Even so, New-Who has set a precedent for the Doctor keeping track of his age, even if it doesn't match up with the Classic series or the expanded media.

Nine says he is 900, Ten says he's 906 when he regenerates. That's too short and too specific a time for him to just make up. Eleven then gives his age as 909 in Series 6, and 1103 after putting off going to Silencio. After this he stops dealing in specifics and his next recorded age is 1200 in "A Town Called Mercy" which he then reaffirms in "Day of the Doctor" with "1200 and something". He then spends 300 years on Trenzalore after sending Clara away the first time ("Where have you been for 300 years?") and then another 600 after sending her away the second (Blurb of this book says he spent a total of 900 years on Trenzalore).

Since "Rose", we know that the Doctor aged approximately 1200 years. How old he was before Rose is conjecture, as 900 could just be a number he pulled out of the air when he couldn't remember, but in terms of the revived series, it has taken 1200 years from his point of view.