When do you replace your pads? Dymoece recommends not letting the pad's braking material wear thinner than 0.9 mm, but it certainly doesn't hurt to replace them sooner than that. My personal rule is to not let the pads get any thinner than the thickness than a dime. You'll also know that they have worn much too thin once the pad spring starts making contact with the rotor. If that is happening you know that you should have replaced the brake pads awhile back!
How to choose bicycle disc brake pads? In my opinion you should run metallic pads if: -You are heavy -You ride downhill -The conditions are wet, muddy, etc.
and run resin pads or semi metallic pads if: -you are light -Conditions are dry -You like lots of initial grab -Your riding is less hard on brakes i.e. dirt jump/street, XC, etc.
Although I have now recommended pads for you to use, you have to make your own choices on pads according to your riding, the above list should be considered to be a guideline, and not used as a strict rule.
Dymoece 2 Pairs Bicycle Disc Brake Pads Compatible with Avid Eli