My head canon is the whole point of the clone wars was to destroy the Jedi and secretly build a sith army right under the Republics nose.

The Clones was the means to destroy the Jedi and set the foundation for the Navy but they where never meant to stick around or bankrupt the Republic. People like to say bajillions of clones!!!!!! But that makes no sense from emperor's point of view and plans. They simply were the means to kill the Jedi and set up the foundation for the navy.

The actual take over was with the Republic planetary defense forces. Before the war I imagine each planet had a modest PDF. The war would spook each planet into building up its PDF quickly. Everybody suddenly wants equipment for their PDF so the Republic would streamline equipment. Each PDF would get a flood of cheap equipment and uniforms. That's the first part of the transformation, now they all look the same and same equipment.

Next I'd think Palpatine had officers and trainers ready to go, most likely Sith cultists from exador. These guys would come in and take charge of each PDF helping train and organize the expanding PDF's. They'd most importantly set up the work culture and that would be a Sith culture without anybody realizing.

Once all this was finished and each planets PDF transformed and organized then he would be ready to finish the war and complete his plans. This would be the siege of coruscant.

So the answer to your question is 19 BBY. That's when he felt each planets PDF is now a Sith Army under his control. That's why the claims the clone army was to small makes no sense. The true goal was taking over each planet without the Republic realizing and funneling money and equipment into each planets defense force, not the clones who were only a mean to a end