Natural discharging duration of an unpluged capacitor
You could characterize your capacitor as follows if you have access to the proper test equipment.

Use an adjustable bench supply and connect its outputs to the capacitor through a microammeter as shown here:



In some cases it may be necessary to use an even more sensitive meter.

Adjust the voltage supply to various voltage levels from near zero up to the upper limit of interest while keeping in mind the upper voltage limit of the capacitor. At each stable voltage step measure the voltage and current and make a table of data. For each data point you can calculate the DC leakage current of the capacitor using Ohms law. The resulting resistance can then be plotted in a graph to see how constant it is with respect to voltage.

Once you know the effective leakage resistance of the capacitor you can calculate the net self discharge time using the standard RC formula from the starting voltage level. If the leakage resistance is constant this technique would be quite accurate. If it is not linear then extra work would be needed to integrate the changing resistance versus voltage into the calculation.