It seesns the the Doctor lost track of their age in their Eighth life. After all, 8 was pretty bad with age, and has said he was 600, 950, 1000, 1129, 1200 among others, but I feel that we can work out how long the Eighth Doctor lived. In Vampire Science, the Doctor calls himself "3", as 3 years have passed since his regeneration. He reckoned that in total he was 1012, but was unsure, so started counting again. My theory is that the Doctor uses that reset from his eighth incarnation onwards, and by using that we can determine how long 8 lived. Given that the Time War lasted 400 years, and by the end the War Doctor was somewhere in his 800's, this indicates the eighth Doctor had around 400 years of adventure before the War (I'm not including his years on Orbis, as he wasn't adventuring during that time), then spends his last years in the War, up until the Fifth Segment of the Time War.
The Howling → 8's age