It depends with electrolyte that you use, for an aqueous electrolyte you can go until 1V.

The potential range has to be determined experimentally, although it cannot be wider than the potential window of the electrolyte. Even the potential window of the electrolyte is dependent on the electrode materials. For example, if you use a Pt electrode in an aqueous electrolyte, the potential window is usually about 1.2 V, but it can extend to a much wider range if you use a glassy carbon electrode. In your case, you can run the CV from a small range and increase the range gradually to  see when there is a fast increasing current following the flat current, at either or both of the potential scan limits. The potential window for capacitive behaviour is only that when the CV remains approximately rectangular. Do not use a peak shaped CV for calculation of capacitance.

Thank you Mr. Ahmed Bahloul  for your answer.

Mr. George , thanks for your help. But i want to when the material showing pseudo capacitance then how could i calculate capacitance from the CV curve (as pseudo capacitor material shows peak in CV curve)?

If the CV shows an obvious current peak or coupled peaks, the material should not be considered as being pseudo-capacitive. I have explained this in my paper which is attached. Nevertheless, for a material showing peak shaped CVs, it can be used for energy storage as all battery materials. In such a case, the measurement and comparison should be based on the specific charge capacity (C/g or mAh/g) and specific energy (J/g or Wh/kg). In such a case, the CV is not very helpful for the calculation, but the GCD (galvanostatic charging-discharging) is more widely used. 

Article (Free Access) Understanding supercapacitors based on nano-hy...

Many many thanks Mr. George for your kind suggestion and for the paper. It will help me a lot.

we can extend the potential up to 1.2 V for aqueous electrolyte but oxygen evolution reaction starts at particular potential that affects the stability of electrode.  

Hence, the potential range should be selected in order to avoid the oxygen evolution reaction.

Calculation of specific capacitance for pseudo capacitor electrode, we have to take  average charge (Q) of complete scan. C = Q/mV,  Where V is potential window

Or take average current C= i/mv  where v is scan rate and i is average current.

Further you can read this article,

Supercapacitor Studies on NiO Nanoflakes Synthesized Through a

Microwave Route

ACS Appl. Mater. Interfaces 2013, 5, 2188−2196

Dear Subbukalai Vijayakumar, I just downloaded your paper, and believe your NiO nanoflakes surely have charge storage capacity, but should have not been considered as capacitive material. The specific capacitance values reported in your paper are incorrect. In other words, your paper has misinterpreted what is pseudocapacitance. You can understand better what I said here by reading my paper given above. 

Dear George Zheng Chen, I agree Capacitance calculation from CV is a supporting study for Charge-discharge.  But this is the correct way of calculating capacitance from CV curves.

This way is only correct if the CV is rectangular in shape. It is not correct for peak shaped CVs. Unfortunately, some authors did so in their publications.

Thank you Mr. George for your firm suggestion. I have also seen that most of the authors calculate the capacitance by the way Mr. Subbukulai suggest for the peak shaped CV curved materials.