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.