Regardless of if there is a standard, that field is one that should be adjustable. A high horsepower, normally aspirated car with low rear end ratio is going to accelerate with greater force than a turbo charged car with AWD. You'd want to be able to set the camera to trigger in either case, but you may not want the trigger point hard coded so low that doing a power brake on the RWD will cause the camera to trigger.
You could take a look at the 60 foot drag slip times of the type of car that you are looking to apply your camera to and get a pretty good idea of how many [acceleration units] the car is accelerating with. Or if you are looking to apply this camera for accident situations, there probably is a standard on when the airbags are set to deploy. The trigger point for the camera will be different depending on the application.
-Rod