I don't believe anyone should know the exact time to sell or hold on, everyone just uses his or her own strategies to make things fell right and can be lucky sometimes and may not, for me i sell when news that leads to the pump gets to half way waiting for closely end can be a loss sometimes so you just need to think yourself most times.
I am agree with you, no one can predict with the exact time when time to sell and when to buy. People with good trading skill only can make analysis and prediction and they could wrong. I think if we invest on good coin and believe will give us big profits, better hold it for long term because we should give project a time to developing their product