What is the average reduction in air pressure for a mountain bike tire? I've noticed that my mountain bike tire looses air, not because of a leak, etc., but just naturally as time progresses.

Does anybody know the average psi loss over time for a mountain bike tire and tube?

Just the average for a 29" wheel.
Continental tire ( they make tubes as well ) states that their tubes loose about 5 psi per week and suggests topping them off on a weekly basis.

This information is not published but they will tell you as much on their support line if you speak German.

For a tube and tire combination, the tube is designed to hold the air in, not the tire and is the point of loss to consider.