Dooku leaves the Jedi Order shortly after the death of Qui-Gon Jinn; the death of Jinn was the straw that broke the camel's back, so to speak. He felt the Jedi were ineffective in combating the evils of the galaxy, and that they were pawns to the corrupt Senate. It was not long after that Darth Sidious approached and courted Dooku to the Dark Side, and one can assume that the two began their Confederacy plans soon afterward. So, the answer is nearly ten years, which is the gap between Episode I and Episode II.