you just multiply your current density (µA/cm²) with the elapsed time (s) and get the capacitance per area (µA*s/cm²). If you then know your mass load per area (g/cm²), you can transform that into a gravimetrical capacitance of your electrode (mAh/g).
If you have two different electrodes in test, you just get the limiting one with this experiment. If you want to examine one electrode, you have to test against a quasi-infinite electrode, like a thick metal foil of the conducting ion for example.