The discrepancy time is only to deter manual bypass of the input and does not affect the reaction time when the device is activated. We always keep this value under 1 second to make it hard to bypass, but otherwise have no set standard. Usually we measure the actual discrepancy when closing the guard at a lazy pace and add 30% to that time. You don't want operators running around the machine to open and close guards and E-Stops that faulted all the time.
If the guards use magnets and they are mechanically located near the hinge of the door, the natural discrepancy time can be very large, so it is best to have the magnets on the latch/handle side of the guard.