1000 was a number.
The Seventh Doctor celebrated his 1,000th birthday while travelling with Ace and Bernice Summerfield. (PROSE: Set Piece) The Eleventh Doctor commented that he was around George's age of 8 "about a thousand years ago". (TV: Night Terrors) When asked if he knew any tricks, the Doctor stated that he'd been escaping for "about a thousand years, give or take a century or so." (PROSE: Magic of the Angels) When he met Clara Oswald, he rounded his age down to 1000. (TV: The Bells of Saint John)