Somebody asked me this and I honestly don't know the answer so will ask it here. When we send off a PM and then later decide to edit, how much time does one have to do it? If you were to come back say 15 minutes later to make a change, would it be too late? (providing the PM hasn't been already read) I realize that the edit window for a post is 8 hours, but I imagine that would not hold true for PM's.
It is definitely not true for PM's. It would be only logical to disable editing when the first participant reads the message, and thus disable the "history" feature. I do not know the exact time, however I sent a test message to an inactive member, and I couldn't edit it after only 2 minutes. It also doesn't seem to correlate to whether the message was read or not. Hope this helps.