cheap and cheerful method
in 1 second the PD drops to about 1/3rd of its original value
the time constant in seconds for capacitor discharge is RC (the product of resistance in ohms and capacitance in Farads)
The capacitor discharge equation looks like
V=V0e^(-t/RC)
its not in the formula sheet, but IMO is worth remembering that in an elapsed time of 1 time constant the PD will be reduced to 37% of V0 (because e^-1 is 0.3679)
and 37% is approximately 1/3rd
so for this setup RC is approximately 1 second