[Matt H.]

The exact solution to your question can be found using the ideal gas equation PV=nRT where n is a function of time based on the compressor's stats.

However, it is easy to make estimate of how long it will take to retract with full force.

Initially your system has 128 cuin of 60psi air stored. 64 of which can be used before the upstream pressure drops to 60 psi and more complicated math applies.
So extending and retracting the piston at full force takes 150.8 cu in of 60 psi air.
86.8 cu in must be provided by the compressor.
Using the 50 psi rate* of .41 CFM or 12 cu in/sec it will take you roughly 7.2 seconds to retract the cylinder at 60psi.

*The 50 psi rate is a good estimate because the system will drop below 60 psi.

Here is an experiment you can do to get an exact time. If you don't have your pneumatics system set up perhaps another team could do this for you.

Steps:
1. Charge all of the tanks to 120 psi.
2. Open a solenoid valve to the cylinder, and start a timer. Wait until the pressure valve reaches 60 psi.
3. Quickly switch the solenoid valve and retract the cylinder.
4. Wait until the pressure valve reachs 60 psi and stop the timer.
Repeat a few times and average the results and bingo you have the answer you're looking for (which I bet will be between 40-140 seconds).