When market tells you so. ATH is often a best time to sell it. Especially if ath happens on bear market
It is easy to say that ATH is the best time to sell, but do you know if it is at a peak or the price will be higher? Nobody knows. So it is easy to point at a chart and say where you should have bought and where sold.
In my opinion, everything depends on your strategy and the coins you have chosen. In terms of shit coins, you shouldn't be greedy and try to return your deposit at least and then you can hold a bit.