Right now, people can edit thier own posts for 10 minutes after originally posting it. I would personally say that is sufficient for the vast majority of posts that are made on this forum. What people really should do it proof read thier posts before sending them to give you all the time you might need to look for and correct errors. I understand what you're saying, especially with regard to longer posts. I also must confess I'm as bad anyone for only finding mistakes after I've posted something, but in the end what I should have done was look harder for those errors before posting it, and that's a responsibility everyone has.
If we extended that limit (especially to the kind of lengths you desire like above 15 minutes), we would invite abuse to the system like somebody changing thier statement to better argue against a point that was made after thier original post. It's much a better habit to have a good look through your post before sending it, and a shorter edit window I believe helps encourage that behaviour.
If it were to change, I maybe would be happy with it at 15 munities as opposed to 10, but anything longer than that and it it could cause more problems than it solves and it wouldn't solve this problem described in this thread as the so called "post farmers" don't care enough about thier posts to re-read and correct them. Not in a 10 minute long edit window, not in a hour long edit window, it would change nothing.