Shimano M495 Disc Brake Pads

When I purchased my mountain bike a few months ago the guy who had it before me contaminated the brake pads with oil. He must of thought the slight squeel needed some oil for some stupid reason.

Anyway that was four months ago and sonce then the pads have been great. However this weekend I was halfway round Llandegla forest when the front and back brakes seemed to lose power all together.

Not thinking it was the pads I took the bike to my local LBS who discovered the Shimano M495 pads I bought were already down to the metal. Ive probably done around 500 miles since I changed the pads which I wouldn't have thought was a lot.

How long do these things normally last as it could get pretty expensive replacing every 4 months, or do you think it was just a dodgy batch??