Take my advice with a grain of salt because I have been studying for 2 years and still haven't placed a real money trade. What I found out is that there are a lot of skills needed to trade Forex.
First, you need to find trade setups. This can be based on price action, indicators, fundamentals, a wide variety of things.
Second, you need to figure out what entry technique you want to use. This could be based on indicators, price action patterns like double tops or bottoms, Fibonacci ratios, support/resistance levels, psychological numbers, basically, whatever you want.
Third, you need to figure out how you will exit your trades. This could be based on many of the same things as entry techniques or you could have a rule that says you have to get flat at the end of your trading day.
Fourth, you need to learn to manage your money so you don't go broke.
Finally, you need to take all that stuff that you learned and build your trading plan. Once your trading plan is built, then you need to test it with historical data to confirm that you have a postiive expectancy. Once you have the historical test done, you need to demo trade it to make sure you can execute the plan under real market conditions.
Once all of that work is done, you can then fund your account and start trading for real. So to answer the question of how long to get to trading real money under just one strategy is however long you can do all of that. Oh, and once that process is done, you get to start it all over again with a different strategy as a trader should have multiple strategies tailored for different market conditions.
Or you can do what 90% of traders do and just dive right in...