How long should it take to change oil pan on Isuzu NPR?

It should take the average mechanically inclined person about 4 hours. You will need to drain the coolant and dis-connect the radiator and maybe heater hoses,other lines and wiring,etc. to gain clearance to raise the engine. You will also need to remove the cooling fan and shroud supports,and unbolt the motor mounts. Jack the engine up and support it,and remove the crosssmember that holds the mounts. Now you can remove the oil pan without interference from the oil pump and pick-up tube.