There is no total allowance to run out. Data keeps working for every day of the plan, and you are never cut off and never asked for more money.
Each day includes an amount that runs at full speed. Once a day passes that amount, the connection carries on at the slower speed the plan states, until the next day begins and full speed returns.
Both numbers are printed on every plan before you pay: how much runs at full speed each day, and what speed it continues at. We do not hide either in a fair-use footnote.