You will know when to sell your crypto investment when you think and calculate that you already have enough margin or profit from the investment. But this is also tricky and could be made you feel frustrated if the price is becoming higher than when you sold.
to sell it you must have a target, if you have reached your target, maybe you can sell it.
do not be too big or greedy for profit, because market movements are unpredictable because you can lose if you are too greedy to continue to wait until your greed is reached.
often check market movements, if your coins have gone up well enough you have to sell them before it's too late and the market can go down again.