when the price goes hyperbolic, when you see new all time highs every hour or every day consistently and it's increasing too quickly too fast, that's when you sell.
the question is .... When exactly ! selling in the middle of up trends is a common thing , but when you in a moment where Many times got the ATH you could wait a little bit longer until it's go down a little bit ... I prefer sell on a correction that's show a bearish rather than have to missed the top value. Altcoins often break the ath a lot of times , that's what I can tell you.
All traders have a strategy to buy on low price and sell it on high price for his profit and we all know that. Like Bitcoin we should also buy Altcoin on low price for getting profit in high market. The process is very simple if one can buy Altcoin with low prices he can collect more than more coins for trading; this is the main and best idea to sell it back when the market price goes high to make good profit.