I think you shouldn't set a limit on how long you use a demo account. It should be based on your confidence and the consistency in your trading. It took me at least 6 months and even now if I have a run of bad trades I will head back to a demo to regain my confidence.
I think everyone needs a different amount of time based on their strategy, how much they studied, and how emotional they are. I would say after 6-8 weeks I'd start to dabble with a live account to start to get your emotions attuned to the fact it's now real money. And just use 0.01 lots.
At first trading need to know about how to design a system what is need for execute in market so need demo. Then how much time to take its up to you. If you sufficient courage to do trade then try to small amount of real for sometimes for test yourself is it realy taking risk or not.
you yourself will feel how much you need to work with the demo and when it will already be possible to switch to real, because if someone tells you a week, and then to real, then nothing will work out, right?