It depends on how strong of an applicant you are. For many schools, they'll extend IIs to their stat superstars very early on in the cycle (UChicago I know tends to do this). For other schools, they review your application in an arbitrary order. So when you receive IIs depends on the precise mix of schools you applied to. If you're a strong applicant and applied to mostly top 25 schools, then you would probably receive most of your IIs in August and September. But for most people, the IIs come in randomly.