In addition to depending on the target, it also depends on which coin is holding. If you are holding altcoins or shitcoins then you should sell them when they hit ATH, as you said every altcoin has its life cycle, not all will survive and evolve over time. But if the coin you are holding is bitcoin then I think you can sell them when the price hits ATH or you can also keep holding for a long time, bitcoin is the only coin that will increase in price and last with time.