Hi,
I wouldn't bother using a PLC.
If I understand correctly, the piston is a two-way piston being driven by two solenoid valves (type 3 1/2)
Depending on the applied air pressure and the valve cross section the piston will stay at the fully extended position for the required 20ms if you reverse pressure to the opposite side of the piston. Reversing pressure doesn't necessarily mean that the piston will move instantaneously. It has to bleed off the pressure on one side first before it moves.
If the piston moves normally at e.g. 1bar and you supply 4bar of pressure, the excessive pressure (3bar) has to bleed off before the piston moves in the opposite direction. The speed of bleed off depends not only on the pressure, but also on the cross section of the pneumatic system (valves, hoses and air supply for the cylinder)
Boncuk