6-8 weeks is a good rule of thumb. Sometimes you can find earlier dates, say if someone cancels and a spot opens up, but then you don't have any flexibility in terms of which day of the week or what time in the day.
Plus, it can be seasonal and vary by location. In my area right now, all test dates are booked until September due to hundreds of teachers testing to get certifications over the summer for different teaching positions this upcoming school year.
