[ARCHIVE ANSWER]
Between 30 to 45 minutes. This is based on a number of civil cases, like Mesa v Ohio. All well-informed/trained officers
know this (particularly state troopers as they would be the ones most likely to suffer a Title 85 suite for a long holding). However, they will only bend this if they know or have the feeling that you know. Hint: go to a used book store and by a criminal procedure 1st year law book and just keep it in plane view in your car.
IT WILL WORK