he would've been in his late 980's
It's heavily implied that the Eighth Doctor regenerated shortly before the events of "Rose". A few episodes later, the Ninth Doctor gives his age as 900. While this does contradict earlier ages such as the Seventh Doctor being 953 in Time and the Rani, we can assume the Doctor was around 900 in the Time War. However, it's suggested by Steven Moffat that the Doctor doesn't know his age so we can't give you an exact answer.