Currently the option to edit posts is limited to 4 hours. We could see about extending it a bit but the reasoning for this limitation is that if a member goes back and removes or substantially changes a post after there have been responses to it, the discussion can be substantially disrupted. This is not fair to those who posted in response.
The 4 hours seems to be long enough to allow for basic edits and after thoughts. It's important to remember that generally folks reading threads don't go back and read posts they've already read to check for updates. So if you have a substantial addition or update it's always best to add another post. If that doesn't work for you, you can contact any moderator and they can make an edit for you.