For me, i dont think there is actually a specific time needed for a trader to trade with a demo account, it mostly depends on the performance of the trader. I have seen cases where some persons send a month with a demo account, while there are cases some use up to two or three months self. It all depends on your understanding of the concept forex trading. I would advice even after getting enough knowledge of the forex market, still make use of your demo account. Demo accounts for trading are not used for learning alone, they are also used by traders to create their personal trading strategies as well, before giving it a try at their live accounts.