BTW, you are partially right with regards to who sets the standards...agencies and jurisdictions do tend to set their own standards (Secret Service does train and qualify once a month with each weapon (pistol, shotgun, subgun). However, the twice a year thing came from a supreme court precedent for a minimum universal standard for law enforcement. Again, can't remember the precedent, so any help is appreciated.
I am pretty sure the USSS qualifies quarterly (4x a year), not monthly.