I don't recall exactly, but for both my kids I know that they were already back to being near the top of the class within a month. Of course, in order to subject accelerate they had had to show that they already knew most of the material, so it was really just a matter of filling in a few holes, getting used to actually having to pay attention to what the teacher was telling them ("You mean teachers actually teach me things I don't already know?"), and getting over the emotional hurdle of not already knowing everything before it is even introduced.
