How long before my bruxism night guard makes any difference to the jaw?

It varies . Some people get immediate relief from pain within a few days. Others take longer. Some people do not get much relief unless they wear it more than just at night. For some individuals, full time use of the guard is needed.
Depends. Everyone is different in terms of how they react to using a night guard. The cause and the duration of your problem is important to know. The type of night guard may also determine success, if at all and the time period for success.
Quickly. For some of my patients, I have heard them say they felt a difference even in one night. The major benefits often occur after 3-5 nights. My patients have commented that they had no idea of how much pressure or pain they were holding in their face and jaws. The right guard makes all of the difference.
Treatment. Most people tend to start feeling a difference in their pain within 4-6 weeks.
3 months. If cause of bruxism is airway obstruction simple guard may be useless. You need to evaluate yourself with your md for obstructive breathing disorder and reevaluate what kind og appliance you need.