For me, I think the best strategy is to set a target. Set a target into when you will sell your coins and when to buy your coins. Use some indicators that will help you in trading and set a target. In that way, it will increase your chance to get profits.
This is the best feeling in doing trading, setting your target or taking profits on your every trade. For me, the term of the OP is something wrong. Taking profits is not quitting, it is the best doing in trading. Taking profits is never a bad decision in trading, it's only sign you are improving your skills in trading once you take profits.
I agree with what you have said, OP might just mean what you have said but the manner of expression or presenting it might be the issue in his statement, I think I understand what he said, his own art of quitting is probably knowing the time that is ripe for an investor to take profit both in long term and short term investment.
For short term investment, like you said, the best time to quit for trader would be when they have met the target they have set when placing an order, but they don’t have to be rigid about their target because sometimes, the market might not be good then, while the best time to quit an investment for long term is when the coin has given enough satisfactory profit or when it gets to its ATH and about to reverse its price.