That will be depend on the leverage that you use, with higher leverage will be promising higher profit but also with higher risk. If you want to jump in to margin trading, you have to learn it first in demo account.
Let's even forget about leverage for now. Making profit from any trading at all, will totally depend on your knowledge, discipline and strategy. A good strategy is not just going to keep you making good trades, but your ability to execute those strategies without any emotion is what makes you a better trader in the long run. Like you said, using demo account is a good start, but no point wasting your time on demo account if you do not even have a basic knowledge and some strategy you want to back test and gain confidence on. There is no fixed ROI when it comes to trading, but making profit is totally dependent on how good you are able to execute each trading strategy to make profit.