Kabuto is 8 years older than Naruto. Now, I am going to say he is 10 when he joins Root, which makes it 2 years after the Kyuubi attack. So 10 Years before the start of the manga.
Time passes, at this point in the flashback I'd say he is 13-14 years old, so 3-4 years have passed. We are at 7-6 years before the start of the manga.
From this point, he infiltrates Konoha, does some missions, lays low, fails the exam 7 times and that's where we meet him.
The surprising part for me has been that Orochimaru was in the Leaf after the Kyuubi attack. I had suspected it, but it is nice to have confirmation.