it really depends on the individual you are dealing with and the project you invest in, it doesn't matter how long you keep it, but most importantly you sell it in the right time by following the market, research and analysis, so that you won't regret or panic selling because prices go down. For me personally, I will sell the currencies that I think are reasonable and I do not regret when the currency appreciates, because it is my decision, manage your risk well and follow the plan that you make, don't be so greedy.