It's hard to tell when to sell the altcoins. Because if it is rising it can be tempting. But at the same time if you hold too long it may crash fast and then you have nothing for a while until it rises again.
It is best time to sell when the price is bigger than the amount you bought it. If you want to earn more you can hold for a long time and check and analyze the date of price of you coin. Then you can make a plan or set a goal of an amount you want to earn for ypu to know when to sell your coin.