I don't know. It depends on how they conquer humans. Let's say, for example, that some of Visser Three's plans DO succeed just one time, that he does succeed in doing something big like infesting patients in a hospital or infesting several world leaders, etc, and the invasion speeds up dramatically. I think they'll import more humans to do more laborious things, like help build ships and weapons. And as Earth gets more under Yeerk control, they'll probably be building in the open, using some of Earth's own resources and our own stockpiles to build even more ships and weapons. I estimate that late in the series, before the end of the war there were maybe a few hundred thousand or million Controllers. Visser Three was indeed a glory-seeking idiot to think open war would work when infiltration is the way to go.
Anyway, with so many ship-building and weapon-building going on, the Yeerks would probably attack the Andalite home world or other key places in about, or, a few weeks or months.