Typically, the decision to sell a stock becomes much easier when you understand why you bought it in the first place.
Every investment in a stock, ETF, bond, or whatever other investment security must come with an underlying thesis. "I'm buying this because of XXX".
Ultimately, the decision to sell should come at the point where the reason you bought is no longer accurate. For example, I own Amazon because I believe them to be a leader in both the retail space, along with the development of its cloud platform Amazon Web Services.
If either of these situations changed, maybe Amazon starts to lose market share to a major competitor or its Amazon Web Services segment starts to report a large slowdown in revenue, it is fairly easy to revisit my underlying investment thesis and if it is no longer accurate, I can sell without issue.
Where a lot of people get caught up when it comes to individual stocks is they really don't know why they bought in the first place. As such, they really have no idea when they should sell.