A good question.
You use a demo account to test your trading system as you develop it, and yourself, into a position to consider the performance is sufficient to justify going in with real funds. Your system is stable, in that you are not changing approach, parameters, system, timeframes, indicators (if used), entry/exit/stop criteria, scale-in/out criteria or any other factors for the last 100 trades. It has become boringly consistent.
You will have gradually upped the risk from a fraction of 1% to a full 1% per trade during your proving period on the demo account.
Any time you get 5 losers in a row, you drop your risk size in half until you get your funds back up to the level they were at before the losing string.
Any time your account drops to 10% less than starting level, you drop your risk size to 1/10th prior size until you build it back up to the starting level.
Regardless of your selected primary trading timeframe I think it is near impossible to get to this level of proficiency in anything less than 6 months. I don’t know anyone who is trading today that did it in less time. Plan on a year to 18 months on demo. It’ll frighten you if I tell you how long (and how much) it took me by NOT taking this amount of effort time and control during the proving phase. You wont get there any quicker by going faster.
When you’re confident in your system, your style, your timeframe and all the other factors that need to be solidly in place before venturing out with real funds, and your P/L is showing consistently profitable returns with manageable drawdown you assign capital to a live account.
You then, and this is key, drop back to a fraction of 1% risk per trade on your live account just as you did when you started on the demo account. You work your way up to a full 1% risk on the live account, taking the same drop-downs on risk when you hit 5 consecutive losers or drop 10% of your account.