I am new to forex and want to start trading but I have no idea about it. So I am going to ask that does demo trading help us to get the required skills of forex? For how much time we should do demo trading and when can start real trading?

Yes with a demo account you can really get some proper training in trading. It personally helped me a great deal. I spent around 2 months training myself on a demo account, and by then I felt rather reassured about trading, and started off slowly with real trading.