How long should MTB disc brake pads last? I'm fairly new to mountain biking and I was able to put in around 900 miles on my first set of brake pads on my Giant Anthem X2. At that point they started grinding metal on metal. I went and bought new Shimano pads (I have Shimano brakes) and after only 100 or so miles one of the front ones has already worn out. 

The last time I rode the bike was in rain and mud so I'm not sure if that contributed to it.

How long do the brake pads typically last? Is there something else going on?
There are different types of pads, even to fit the same model of brakes. 

Sintered metal pads last longest, but are noisiest, often to the point of not being worth the better durability. 

Resin pads are softer, and will last less mileage, particularly in wetter weather, but still should last far longer than 100 miles. 

I would say that the rotors are damaged from being ridden metal on metal before, and need to be replaced, along with new brake pads. 

Your mileage will vary based on weather, braking habits, pad type, riding style and terrain. But you should normally get 500-700miles from a resin pad, and 1000-1250 miles from a sintered metal pad.