My airline (regional) has a guarantee of 75 hrs/month. Each Mil day not available to the airline takes 2.5 hrs away from the 75, for reserve lines. I think that's pretty typical.

Once you gain some seniority and are holding an actual line, it gets more complicated. Some of your scheduled airline trips can be "pay protected" by the airline, depending on how it's scheduled. With a hard line, you can also be more selective about where to drop mil leave in order to maximize your pay. For instance, if you work your mil duty on days not scheduled at the airline, you incur no loss of pay..just loss of family time..