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I’ve seen this commented on threads such as
You are searching too quickly. Reduce search 90 seconds limit?.
This has to do with your current Activity, and the response time improves noticeably as this parameter increases. Below is the algorithm being used (second quote). Although it is often refered to in a posting context, it also applies to a search context.
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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);