Matt, I think there is some use to a historical record on these boards. Haven't done it often here, but on other forums I'll go back and work back along a thread for what may be months. If some of those posts are deleted, it breaks the thread and you don't know what the sequence of thinking was. So I vote for the shorter, rather than longer, time limits, with you having the ability to dump any post that needs to be if something really wrong escaped the moderator's attention. (Any moderator who can't be on most days should get assistants.)
If some post becomes inaccurate by passage of time, it's always possible to go back and reply to it even six months or a year later. So it's not as though information will stay on the boards that can't ever be commented on or refuted.
In general, though, open history is better than revisionist history.
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Funny thing, but it seems that the harder I work, the luckier I get.