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The message is somewhat misleading, as some other events also count within the given timeframe (i.e. searching and some other events).
I can’t recall the post I saw this assertion, but the 360 seconds (or other) is not strictly down to PM events, despite the message.
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Q: Why do I get the "The last posting from your IP was less than 360 seconds ago." error when I haven't posted today?A: Logging in, sending PMs, searching via the forum's built in search engine and reporting posts to a moderator will also be counted as posting and extend this limit back to 360 seconds. Don't worry: as your activity increases, the 360 second limit will decrease as well. Here you can see the algorithm how the posting limit is calculated based on your activity:
https://bitcointalk.org/index.php?topic=237597.msg4131557#msg4131557<…>
In general terms, the waiting time improves as you gain activity:
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waittime = 360;
if(activity >= 15)
waittime = (int)(90 - activity);
if(activity >= 60)
waittime=(int)(34.7586 - (0.0793103 * activity));
if(activity >= 100)
waittime = max((int)(14-(activity/50)), 4);