As far as I remember the SAE standard is 60 to 120 cycles per minute, and a bulb failure is supposed to make the system increase the rate to more than 120 so that the operator knows a bulb is out. As far as police and road safety, I don't think there is a number, but they could make a case by saying "as it was when delivered from the factory". Or they are just looking for a reason to write a ticket.