According to the ECMA specifications, the maximum PIF per 1 ECC should not exceed 4 when reading at 1x speed.
First of all BenQ drives count PIF per 8 ECC blocks (not per 1 ECC block) and secondly they read (scan) at higher than 1x speed. This means that an exact translation of the ECMA spec is not possible, but if you have a maximum of around 8-12 you probably have a good burn.
Personally I re-burn whenever i get an ugly Read Transfer test in my NEC drive or if there are significant amounts of PIF values exceeding 5 (per 1 ECC block) when scanning at 2x speed in my Plextor drive.
An exception is when I burn DVD+RW media, because I can NEVER get those as low as 4-5 PIF.
EDIT: And of course I always re-burn if a scan shows POF (Parity Outer Failures)!!!