The circuit you posted is a flyback inverter circuit. The maximum voltage that will appear across the 0.68V capacitor depends upon the maximum current through the inductor just before the transistor turns off. Then the stored energy in the inductor is transferred to the capacitor at the peak of the resonant cycle. The peak inductor energy is 1/2 LI² and the peak capacitor energy is 1/2 CV². Equating them and solving for V gives V = √(LI²/C). The supply voltage does add to this voltage so the total peak capacitor voltage would be Vpk = √(LI²/C) + Vcc.