It's not really clear what you're asking, since forex tends to be a field for short-term trading, not long-term investment.
A traditional rule for the balance between investments and trading funds is that the percentage of your capital in long-term investments should be greater when you are younger. So if you are 20 years old, 80% of your wealth should be in long-term investments: if you are 50 years old, then 50% of your wealth should be in long-term investments. Of course if you have been investing since you were 20, by the time you have reached 50 those investments should be much larger by then. Also by the time you are middle-aged, some invesments may have been converted into less speculative forms of investment - for example, from stocks / shares into property.
I don't know of a viable multi-year forex investment, just don't stop trading.