You have two minutes after posting your comment to decide you need to change something.

Once you click the pencil icon and open the editing dialog, you have five minutes from the time your comment was posted to submit your changes.

I’m deliberately keeping the time limits rather short, because the idea is to quickly fix a typo, not rewrite an entire comment.

Also, once your comment is posted, everyone who has already loaded it in their browsers will continue to see the old, unedited comment until they reload the page. There’s no easy way to force all the browsers out there to reload edited comments, unfortunately.