When is it time to sell a crypto?

It really depends on the reason why you buy crypto. You can use it as money and pay for things, or just store it as a value for the long run. If it is bought as an investment it all depends on your asset allocation rules. If you can only hold a certain percentage of crypto, you have to sell automatically if you go over this threshold. This serves as an automatic profit taking mechanism.
Another rule could be: Sell 50% of your holding when you have doubled your money. Then let the remainder ride until you either need the money or you have reached your asset allocation threshold.
Another idea is to take profits when the real interest turns positive (interest of 10 years treasury bond minus inflation). You can set this signal also when you already took profit after you doubled your money. This is a very uncommon advice but we are aware that the crypto rally is fuelled buy a massive amount of cheap money. As long as the real interest rates are negative, the money will continue to flow. Should the real interest rate turn into positive, this development might start to slow down. This would be a good chance to reassess your holdings.
Do not use trailing stops as you might do for your equity positions. Cryptos are too volatile to use traditional stop strategies.

Sven Franssen