In my opinion, the best time to sell is when the market price of the coin is 20%+ higher than the market price at which you bought the coin.
Yes if you invest and hold over a longer periods of time. But if you day trade, the margin for profit can be much smaller. Even a gain of 0.5-3 percent can make a difference if you are good at day trading. Experienced trades can make a good profit even with such small profit margins.