In my opinion, the best time to sell is when the market price of the coin is 20%+ higher than the market price at which you bought the coin. Dont wait till the market price is 95%+ before you sell. That only happens when you got your eyes glues to your machine 24/7. Dont be greedy... Make a little profit and grow it over time. Cryptomarket is growing and full of terror! Be careful
Everything starts with strategy choices: short-term, medium-term or long term.
If you choose long-term strategy the 20% gain is to small for example Bitcoin in the bull market usually goes 300% or more. But I am agree with you if you choose medium-term strategy and you want to be a careful investor. Then 20% gain is pretty safe choose.