The main reason traders are attracted to Forex is profit. Can I achieve +100% profits in a year with this particular system or method? Practice will make perfect and will allow you to find out if the trading strategy is in agreement with your own style and personality.
Now, any method is much less important than the trader him/herself, how does he/she apply the technique, which timeframes are used, how many hours a day will be devoted to trading, what risk level can be afforded, and so on.
100% per annum (assuming you do not compound the profits) is around 8% monthly which is an achievable goal in the Capital Markets. You could make more, you could make less. But in the end the decisions are yours, when do you enter and exit the trades, how outer elements can influence your state of mind, how do you handle money management and risk, how do you cope with a losing streak, etc...
We all have some kind of goal as to results, but the most important part in my opinion is to focus on the task at hand, strive to trade every day better and follow the system steps as well as your own trading rules and plan. A system or method is not quantifiable by itself unless it is employed in an automated form; thus, as long as you are a part of the equation, the final outcome will highly depend on what you really want to achieve.
Forex trading has a way to take everyone down to the innermost details of one's psychology, revealing all of our strenghts and weaknesses. Consistency, discipline, patience, ability to follow one's own rules, recognizing the assets that we possess as well as the failures we're bound to experience, and above all never forgetting that, in the very end, it is Mr. Market who leads the game, are indeed key issues. If you are humble enough to accept what it is willing to give you when it does give, you'll succeed.
How much does that represent in terms of percentage of your account will depend on your money and risk management and thus can't be quantified, but… focus on the road, and the destination will come to you!