If you are a mere trader, then I can tell you this in all honesty that it is time to sell your crypto when the prices are going astronomically high (like they did last December). I know that I am going to catch a lot of hell for saying this but truth needs to be said.
You will notice that the the selling pressure has evaporated gradually over the period of last few months and now is the time to buy. The opposite of this will happen too. At the end of a bull run the buying pressure will eventually taper off and then the selling would begin. A smart trader would sell before that happens. The fact is that it is time buy when we are oversold (like now) and time to sell when we are overbought (like last December). That being said, it is important to remember that panic selling is much different from a calm and calculated selling decision.
As always, I will also say that a person needs to be sure of what his position is. You need to decide whether you are a trader or an investor. A trader would take his profits where he can but a smart investor is not be concerned with just the profit but also with the quality of the product he has invested in. An investor who believes in his investment would rarely sell.
Crypto is such a new phenomenon that we do not even fully understand its potential but one thing is for sure - crypto would change the way of how the world understands money. It is my personal opinion, but I think that crypto is going to change the world and if you have invested in a good project than the only good time to sell is perhaps after 10-20 years.
NOTE: This is not an investment advice but just my opinion.
Right after it has rallied ferociously. Cryptocurrencies tend to rally explosively and take their time correcting in stages. If you're going to trade, I suggest you study technical analysis and how it specifically applies to cryptocurrency.
There's this general quote in the crypto space "buy low and sell high". This the answer to your question. The best time to sell a crypto is why the price has risen every high that you can make a considerable gain on your investment.
Most people crypto investment strategy is to buy low, hold for it to get high and sell. That's how most big crypto investors amass some great profit. So if you want to sell, wait for it to climb higher.
Buy low and sell high. I have learnt to always sell when the coin is on the up.
Most traders call it laddering and it is used when you buy and sell. When you sell you have an idea of where you are comfortable selling and sell in stages forming a ladder effect.This way you don't feel resentment for selling too soon and average out a better selling price.
Most investors would rather hold as the crypto market is still in its infancy and has only just started. Day traders sell on a wave pattern buying the troughs and selling the crests. it is amazing how this pattern plays out on a regular basis.