voltage across capacitor at any time is given by V(t) = V(e-t/RC)
So first u need to know about rating of supercapacitor and discharge time constant.. following link may help u to know time
https://www.circuits.dk/calculator_capacitor_discharge.htm
Discharge :
Note: If using a separate programmable load instead of an integrated bi-directional power supply, disconnect the charging power supply prior to discharging. (Don’t simply turn it off or change its set points, as many supplies will sink current when not regulating.)
Set the load to the appropriate constant current, and discharge to 0.1V, or as low as the load can be controlled.
IMMEDIATELY remove the load once the minimum voltage is reached, allowing the device's voltage to "bounce" back.
(The discharge can actually be stopped at any voltage. Depending on equipment, some units can be discharged to 0.1V, and others discharged to ½ of the initial voltage. Values of capacitance will be slightly higher when discharged to ½ initial voltage rather than 0.1V.)
Measure the following parameters:
Vw = initial working voltage Vmin = minimum voltage under load
Id = discharge current Vf = voltage 5 seconds after removal of load.
td = time to discharge from initial voltage to minimum voltage
Capacitance calculation:
Capacitance = (Id * td)/(Vw – Vf) = (Id * td)/Vd
(This change in voltage (Vw – Vf) is used because it eliminates the voltage drop due to the equivalent series resistance)
Equivalent Series Resistance (at “DC”) calculation:
ESR = (Vf – Vmin)/Id
(An LCR meter or bridge can be used to measure ESR at higher frequencies. The ESR at frequencies up to 100Hz will typically be 50-60% of the “DC” ESR. The capacitance will be much lower, due to the structure of the electrode.)
below attached fig.: The circuit in Figure uses the TLV3011 solar cell because it integrates a voltage reference, draws only approximately 3-μA quiescent current, and is an open-drain cell so that the output is open-circuit when the regulator is off. This circuit uses the BAT54 diode because it has a low forward voltage at low currents—that is, the forward voltage is less than 0.1V at a forward current of less than 10 μA.