The edit time-window is there for you to correct spelling mistakes or suchlike that you only really notice once you've hit 'Add'.
If the edit period were indefinite this would open up the possibility of abuse.
Take this hypothetical example. A comment is left against a post by two people:
Billy: "This is a great post"
Jenny: "Thanks Billy, I agree"
Two months later Billy could come back and edit his comment so that it now shows:
Billy: "Jenny is ugly and smells funny"
Jenny: "Thanks Billy, I agree"
OK, ignoring the fact that these are useless comments and would probably have both been removed anyway - it does highlight the possible issues. Also, comments don't bump questions up the question list, so such edits could go unnoticed for months at a time.
Also, comments don't have an audit history, there is no way of knowing what the comments originally said.
Finally, comments are second-class citizens anyway, you shouldn't expect them to stick around or be particularly useful in the long term. There there for requesting clarification or for expanding the issue, but any points made that are important or relevant should either be edited into the question / answer itself or should be left as new posts in their own right. As you say, you were making a >600 character 'comment'. That's quite substantial for a brief comment.
If there is a big problem with a comment then you can flag it for moderator attention, and moderators have the power to edit or delete comments as appropriate. However it's more likely that the moderators will just delete the comments rather than start faffing about with editing them. As I say, comments aren't really that important in the grand scheme of things. Flag comments that are wrong / abusive etc, otherwise just let them be.