knowing how much and when to sell is as difficult, maybe more so, than knowing what to buy in the first place

maybe the best way to approach this problem is to only ever invest in things that you will never want to sell.

some factors:

the expected time horizon going into the investment?

the expected return: is this a % play or a 1000x play?

whats the relative investment to current portfolio size?

what new information has surfaced since making the original decision? 

and then add in the chaos factors of emotion and market noise.