There's never been anything to suggest that time wars have a set length. In the case of the Last Great Time War, the only indication of passage of time we've been given is the fact the War Doctor is dark-haired and relatively young-looking following the regeneration in "The Night of the Doctor" and elderly at the time of the war's resolution in "The Day of the Doctor". Even then, that's relative, given his ability to time travel. Also, in "The Innocent" (Big Finish audio story), the War Doctor was exposed to a Time Destructor, which (as seen in "The Daleks' Master Plan") has the effect of causing extremely rapid ageing. That means the War Doctor's apparent age can't reliably be used as a measure of the passage of time.