First factor that makes me decide to sell is when i badly need the money and no other source of funds for that urgent needs, 2nd is when the price reaches high to the moon and you are already satisfied of the increase of the money you invested, thats the time i may decide to sell and wait again for right time of buying for another holding...
Whether price is on the moon or at the dip, that must not matter, what should and must matter is the target. Anyone who is investing money in the crypto currency, has some vision, mission and target. I suggest is to sell it when you have achieved your target and if you want to stay in the market, then set another target from that point. By doing this you will get a sense of achievement as well as contentment.