The mat will wear out most quickly if you cut the exact same die in the exact same spot. you need to move the mat slightly for each cut, flip it end for end and use both sides. You can try putting a piece of copy paper between the fabric and the used cutting mat and see if you can get some more cuts. If you have cut bunches of the same die, the mat can sometimes be used with a different die because it will be cutting in a different area.
I would say that the mat would need to be replaced as often as you would replace a rotary blade. You should always have an extra mat on hand!