This is a judgment call. There's a variety of reasons to add a Content Warning to your post.
Appropriate use of the Content Warning feature is as much an art as it is a science.
Note that adding a Content Warning for sensitive content is a rule on Infosec.Exchange, but there are other reasons to use the Content Warning feature.
If you polled 100 people at random - either from Infosec.Exchange or from society in general - how many of them would say “ugh I did not need to see that”? If you suspect the answer is “more than a few” then most likely your post belongs behind a Content Warning. Hey, maybe you don't even need to post it.
Content Warning is designed to give people a chance to avoid content they do not wish to see.
Use of a Content Warning in no way provides a magical bypass of other Infosec.Exchange rules and/or basic human decency.
A non-exhaustive list of topics where people may be grateful for a Content Warning for sensitive content:
I shall not today attempt further to define the kinds of material I understand to be embraced within that shorthand description; and perhaps I could never succeed in intelligibly doing so. But I know it when I see it.US Supreme Court Justice Potter Stewart, Jacobellis v. Ohio, 378 U.S. 184 (1964)
Spoilers are called spoilers for a reason, so try not to be the person who ruins something fun for someone else. Use the Content Warning feature to prevent inadvertently exposing readers to spoilers (e.g., discussing the answer to a past CTF challenge, discussing the ending of a movie, book, or video game or anything relating to a live sporting event - not everyone is able to see it live).
Some people choose to use the Content Warning feature to hide content they may think will only be interesting to a few followers, either because it is off-topic vs. their usual posts, or perhaps because it is personal in nature. This is voluntary, and not everyone chooses to do this.
Many blog home pages show a brief introduction to articles that helps the reader understand what the post will be about and to give the option to click if they wish to read more. If you wanted to do this, you would make your initial sentence the Content Warning (even though it's not actually functioning as a warning in this usage), and then the longer text would go in the normal post content. This helps prevent filling timelines with lengthy posts.
There is no official “rule” on what a Content Warning can (and cannot) be used for. Have fun, be considerate of others, don't be a jerk, and experiment!
When using a Content Warning (CW), only the hashtags that occur within the CW itself will be searchable, not any that appear in the (initially) hidden content. As such, be sure to include relevant hashtags to help others find your post in the CW message itself.
There's no rule on this. Some people may disagree with your use of Content Warnings if it is tedious, repetitive, or misleading. If it's egregiously misleading or abusive, you might be breaking one of the Infosec.Exchange rules, but otherwise, it's between you and whoever is reading your post.