There are several variables that go into the decision such as material fastened, bolt grade, thread diameter, thread pitch, etc but a good rule of thumb is thread engagement should be 1.5x thread diameter. I usually shoot for 2x. Fine threads are stronger and can get away with less than coarse, if going into aluminum I go for 2x -3x or a steel insert, and on and on and on. One of the biggest variables is who is going to be the next monkey on the torque wrench and does he give a crap.
