The invigilation centre should schedule the tests in time to meet your prospective senior school's deadline.
Each senior school has a different deadline for when they want tests completed. If you are applying to multiple schools, then your child should take the tests before the earliest deadline.
You can either find out the deadline from the senior schools directly, or, once you have registered and sent your Applicant ID to the senior school, their deadline will appear when the centre schedules the tests in the Invigilation Centre Portal.
The Common Pre-Tests can be taken between October and June of each academic year.