Unknown. The Second Doctor said that Time Lords can "live forever, barring accidents" ("The War Games"), however "The Tenth Planet" and "The Time of the Doctor" both saw the Doctor regenerate from old age. Given that Time Lords are generally only granted 12 regenerations and 13 lives ("The Deadly Assassin"), that means that, given time, a Time Lord would eventually regenerate due to old age 12 times and finally die once the last body wear out (barring an extension to the regeneration cycle like that awarded the Master and the Doctor). The First Doctor lived about 450 years, while the Eleventh lived for well over 1000 years ("The Impossible Astronaut" established a 200-year time jump for the Doctor, while he spent at least 900 years on Trenzalore in "Time of the Doctor"). So a Time Lord's "barring accidents" life span could be in the tens of thousands of years.