Because so many trader fail less than six month to maintain account, almost trader get margin call account in real trading less than six month and spent time to learn in demo at least six month is good than only a week
What I have realised is that most trades usually know when they are ready to go for live trading. You cannot just say that you are going to do demo trading for a certain time period and that after the time elapses you will move into live trading. How well you can trade is not determined by how long you take to practice demo but what you know out of your demo trading.
You have to trade in a demo account until you are sure that you are ready. There cannot be a specific time of how long people are supposed to do demo trading because they are all different. How you are going to handle trading is not the same way I am going to handle trading. You have got to know what is right and what is wrong and your gut will tell you when you are ready.