Even after 16 hours, I was able to edit a post on a blog. Not sure if there is a limit here on the forum BUT allowing folks to edit messages during a discussion could cause issues when someone does so and turns around to say "I never said that"
EDIT: Alright, I was able to modify a post I made three days ago.
We are using off-the-shelf software as a base, with certain in-house plugins and many in-house bugfixes/changes to bring it up to BioWare standards. We still have moderators, plus some nifty new reporting/banning capabilities. If somebody is being a total jerk, we can still ban their account/email address/IP/country/etc from the site. That's why there is a "report this user" button on everyone's profile.
Jassper: what your talking about sounds suspiciously like a wiki; if it's something that will be edited lots, and you want to have a history, then put it in a wiki.