Backstroke it the only one of the four competitive strokes that does not automatically force some level of control on your breathing. The sequence of each stroke provides a built-in moment where your face is in the water creating a regulated breathing pattern. Backstroke’s face out aspect allows for uncontrolled breathing, this I not a good thing. You need to train with a disciplined breathing pattern. If you fall into panting your speed will be adversely impacted.
There is not one pattern that is absolutely correct, it can depend on the skill, distance being done, and training level of the athlete. As a starting place try breathing in for one full stroke rotation, and then breathing out for one full stroke rotation. From this baseline you can change to find the best pattern for your performance.