You will be charged for the additional days in which you stay in the parking lot, based on the ONT Parking Lots & Rates | Ontario Airport Parking (flyontario.com) applicable to that lot. The excess parking fee will be payable upon exit of the lot.
Please note overstay charges are only applicable if you leave the lot 6 hours after the departure date and time entered at the time of booking. In other words, you have a 6 hours’ grace period for arrival and departure to the lot.
Link to the overstay rate can be found here: ONT Parking Lots & Rates | Ontario Airport Parking (flyontario.com)