In FX it is possible to make 100% monthly or more. The hard part is to keep up that kind of performance on a consistent basis while keeping the risk relatively low. It is almost impossible but it can be done. More realistic target would be 5-10% monthly return on average. It also depends how much a trader is willing to risk, bigger risk bigger reward.
In any case, the most important thing is to be consistently profitable. Anyone can make 10-15% one month or maybe even few months in a row but the question is if you can do it each month without suffering large DD.