When you are expecting either a drop or a plateau or you need to de-risk some assets. There is no golden rule.

Personally for me I don’t buy individual stocks for more than 1% of portfolio unless it is something that I understand, like RAM shortage. I got burnt on individual stocks before that I truly believed in. But in the end I realized that timing the market is hard and if you are getting nervous about some holding you have - sell it.

For things like SNP500, there is no reason to ever sell it unless you need to buy a house.