You should stay on demo as long as you understand that you're ready to take a risk. There is no simple answer, like "stay 2 months", it depends on your innitial skills and your mindset. One thing I can say for sure is that you shouldn't stay too long on a demo, there will be okey if you still until you make 12-15% profit from your demo deposit. I.e. if your demo deposit was $10,000 than after you make $1,200-1,500 as profit, thats okey to move real trading and try there. However, you still can use demo account to train and test strategies.