3 Years and 6 months using technologies that he had found from the obsoleted borg ships. He had found one certain tech that has helped him cut what would be a 50 + year job down to a 3 + years job. In Borg Wars you see a fine demonstration of it when a cube completely assimilates a planetoid within minutes. That device is a much more advanced version of the device that had helped the guy.
You see the device that I speak of in this episode and the second episode
YouTube - STAR TREK: BORG WAR: ACT 1