For those of us who like to post long-form things, manifestos, or other thousands-words long posts as a general rule, there's no automatic "read more" cutoff point to a post. Instead, you can manually decide where to put the "Read More" spot in your entries using the <cut> tag in your entry. (It's called that because it's a holdover from LiveJournal which used <lj-cut> to do it.)

The standard cut tag will default to "Read More", but you can customize the text of what you want to display there with <cut text=text>

This only works for entries, and it creates a link to the spot on the entry when you click on the cut link. <details>, mentioned above, works on both entries and comments, so it's up to you to choose which one you want to use for your entries.