750 was a number.
The Second Doctor claimed that it had been 750 years since he had left his home planet. (PROSE: The Power of the Daleks) While travelling with Sarah Jane Smith, the Fourth Doctor told her that he was "something like" 750 years old, making him "middle-aged". (TV: Pyramids of Mars) He later gave his age as 750 shortly after Leela joined him as his companion. (TV: The Robots of Death)
In a letter, the Fourth Doctor claimed to be "over 750 years old". (PROSE: A Letter from the Doctor)
750 years into the Siege of Trenzalore, the Eleventh Doctor defeated the Mara when they invaded the town of Christmas. (PROSE: The Dreaming)