If I edit my answer/question/comment fast enough, it doesn’t say “edited 10 seconds ago by EvilTwinNeedleTM”. Why is this? Also, what’s the time frame for this happening?
If you edit posts in the five minutes after adding them, they won't be indicated on the post. (However, they do show up in the edit history if you enter the post's ID into the URL manually.)
Any edits you make within five minutes of making the post probably aren't going to be substantial: just minor revisions of wording, or correcting grammatical errors. It's unlikely to be worth bumping the post for them, nor drawing attention to the post in the absence of any actual new content.