Honestly, there is no certain answer set to the stone about trading, as there are a lot of coins and different prices, different devs and different interest for each one. in general, you should check the price all the time, check the charts to know the lowest and highest point, buy in the dips and set your % profit, like 5-10% to sell the coins. But, if there is another news/information/prediction if the coin will rise further, then hold it for a certain time. For example; we know that bitcoin ATH is $5000 and its price was $3500 few days ago, buy it and set % of the profits, even sell it right now will be profitable, but hold it for long term until it reach $10K will be more valuable.