cool - you caught the answer before i realised there was a much simpler (and more accurate) solution.
The problem with my first response is that the 5 minute sample would 'snap' to 00, 05, 10 , 15 minutes past the hour. So if your search ran at 09:12 (for example) then you'd be looking for > 50 hits in only 2 minutes.
The updated answer (with now()-300 ) looks back 5 minutes from the time the search was kicked off, and is a far more accurate sample window
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