What is the average reduction in air pressure for a mountain bike tire?
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.