The main idea is that trader should be able to be consistently profitable before switching to real account. That is why it wholly depends on trader`s perfromance and trading results and could not be specified as "1 month" or "1 year". If there is no consistent positive results using demo account, this means that trader could not earn money even in almost ideal trading conditions without psychological pressure, fears and greed, so how could he earn money at the real market?
Another point is that switching to the real account does not mean that trader should forget about the demo. It still could be a perfect tool to improve trading skills and habits, or to test new ideas. At the same time, if we are talking about checking new strategies, it will be better to use special software, the backtesters - it will make the testing process easier and the results will be relaible. Anyway, it is important to test each new strategy on demo to check the influence of the trader`s psychology.
So, use the demo account only unless you will become consistently profitable trader and then use it from time to time to improve your trading.