600 nA is 'nothing', so it's likely that leakage (internally in capacitor, or elsewhere in the circuit) will be a dominant factor.
Just get a sample, charge up to nominal voltage & leave it out for a few hours or days with nothing connected (both terminals in the air). Then check voltage with plain digital multimeter, and you'll probably see that voltage has dropped significantly. Read: only suitable for spanning hours, perhaps a number of days. Very probably not months... :|