If you would have asked this question to someone decades ago, they would have told you to hold on to precious metal investments for a very long time to make money. Today, precious metals continue to grow at very fast rates. You may be able to make some money of holding them for as little as 6 months.
I am not an economist, but I think it is wise to hold on to precious metals for long-term right now. There is little indication that the value is going to decrease any time soon.