Both the BPA and the IPC operate "grace" periods at the beginning and at the end of parking. You can use 20 minutes 'overstay' as being reasonable as a starting point. Anything over 20 minutes becomes more and more difficult to defend.
Grace period at the start
Time at the beginning, is to allow the driver to park, get out of the car, read the signs and decide whether they want to stay or not. This period can be as little as 5 minutes or in one case, 30 minutes, as a Judge decided in a ParkingEye case for a very busy Newquay car park during summer.
The IPC Code of Practice for example says:
B.15.1 Drivers should be allowed a sufficient amount of time to park and read any signs so they may make an informed decision as to whether or not to remain on the site
Logically, drivers are expected to read signs when they park and if the signs are there in number and are prominent, it would be crazy to claim you did not read them.
Conversely ParkingEye start timing their parking periods from entry to the site and not the time of parking. Claiming overstays when their equipment cannot actually measure a stay is crazy too - but they get away with it unless you challenge them.
Grace period at the end.
Again there is a period at the end to allow the driver to leave the parking space. This is almost always 10 minutes. BPA members are obliged to use 10 minutes while the IPC are more vague. Their code says:
B.15.2 Drivers should be allowed a sufficient amount of time to leave a site after a pre-paid or permitted period of parking has expired.
Though they may be vague about it, their largest member Vehicle Control Services uses the 10 minute rule as this IAS appeal below shows.