When you are comfortable with your profit or better still, when you have decided from get go the target price for your exit from a particular trade, that when you should sell. However, there are market conditions that could make you sell earlier or later than planned.
Well i decided to sell when the price of the coins i hold will pump really high, and i also plan to sell the market is green and i plan to sell my coins when i am sure that i can make now a lot of money like what happen last december 2017, i sell my bitcoin immediately when the value of it is pump to $19k.