Sorry, Stack Exchange, but some people type slow and might need more than 5 minutes to type out a comment. How can I post a comment to an answer to my question, for clarification, if the site's policy won't let me do so?!
You can take as much time as you need to write your comment. However, once you've saved it, you only have a 5-minute window by which you can edit it. This allows you to fix a typo you've spotted right after you've saved it, but doesn't allow for changing the content of a comment later on, when there may have been a response.
Traffic is pretty light here, so your best bet is probably just to delete the old comment and try again with a new one.