Nick
Good comments.
Not really an answer though, I'll quess that's because there is no absolute answer. If I read you right you should use the second bottle in about ten seconds - thats fair, given the quick reaction of the S76 detectors. The scoop on the side of the engine cowling most also do it's best to blow away all the extinguishing agent rather quickly.
Speaking of the scoops.
Why was the Arriel engine cowling scoops air direction changed from the Allison, to get better function of the extinguisher? or just because of a diffrent heat source?