The best time to sell not only altcoins bit all kinds of cryptos is when the price goes for 25 - 30% of gains then buy again when the price drops by 20- 25%
In words, it is very easy to stay because when we are in the market the fluctuation of the coin makes to deviate our investment into others when we made the profit. But after our purchase again the market starts going down and I have experienced this type of situation many times.