The best value depends on the dynamic content you have:
- If there are widgets which should be up to date each time, you cannot use a long value, or your visitors will get outdated content. That’s often not a problem though.
- The value should not be longer than your average post frequency. If you add a new post two times per day, you probably want to show that your visitors early enough.
- If you enabled comments, you should use a low value, at least for posts. Otherwise your visitors will think their comment wasn’t sent, because they will get the unchanged page after submitting a comment.
I think, the recommended value is the one that avoid too much support questions. Most users will not even notice the cache validation requests, but delivering outdated cache files is much more visible.
You should run some experiments and increase the max-age value in small steps until you note side effects. Start with 30 seconds, then double the value each week. I don’t think there are important SEO effects. Other factors are much more important.
For a project a long time ago I developed a dynamic value:
max-age = (now - last-modified) / 2
But you cannot do that in a .htaccess file, because the server doesn’t know when a post was last modified in the database.