On the 744 it largely depends on how long the pump is left running, but a good guide is pump on for a minimum of 30 secs. after the park brake is set.
Presuming the brake accumulator is to the correct precharge and the anti skid surge accumulators are also precharged correctly, after 10 mins. brake pressure should not drop by more than 275psi. That would give about 8 hrs of effective park brake pressure.
As CJ says 'put in the chocks'.