There really is no definitive answer. As I commented on in another thread, I normally look for a pattern before I file a SAR. Others will file if they feel it is structuring whether it be the first or XX time.
Its your job (assuming your the BSA Officer) to make that decision. I will offer this bit of advice. No matter what you do, make sure to be consistent with your filings. I have had regulators from the FDIC, OCC and NYSBD all state that they are looking for consistency with SAR filings.
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"100 victories in 100 battles isnt the most skillful. Subduing the other's military w/o battle is the most skillful." Sun-Tzu