If you exceed the thresholds of 10GB or 15GB (depending on the network you're on), speeds will be controlled on the day of the month the threshold is exceeded and will continue for the rest of the billing cycle.
For example, if your billing cycle starts on the 8th of each month and you exceed your threshold on the 25th, you will be subject to controlled speeds from the 25th through to the last day of your billing cycle, in this case, the 7th of the following month. The data counter will reset at midnight that night.
Your billing cycle can be determined in the Wireless Charges section of your bill. The range of dates for charges is your billing cycle.