You will receive warnings by email, and as a pop-up and in the Billing & Usage tab in the console at 70%, 90%, and 100% of the allowable data operations. Once you reach the 100% limit you have three days to switch the the Growth plan, or data will be stopped until the end of your billing month. It will automatically resume on the free plan at the beginning of the next billing month, still on the free plan, if you do not upgrade.