Answer: Sell when there is something else better to buy.
Something else better for future returns.
Something else better for safety.
Something else better for timeliness or fit with today's go-forward worldview; a megatrend.
What is that something else?
It can be:
- another stock
- an index fund
- a house
- or any kind of investment
It can also be cash.
Sell that stock when .... when what? When cash is a better investment. Or when you need the money, which is another way of saying that cash is a better investment - at least it is safer for the time being.
Best possible deployment of your capital
If you think of a buy decision as a best possible deployment of capital because there is no better way to invest your money, you will also come out ahead.
It really is not hard, especially if you have done your homework.
And it is also made easier if you avoid rash overcommitments; that is, you avoid buying all at once in case you have made a mistake or in case better prices come later down the road.
Buy and hold quality companies for the long haul, and you will likely be handsomely rewarded for that patience.