typically, pads have a wear indicator (metal) which will squeal against the rotor when the pad gets worn down.
pads can sit for a while, won't go bad. i guess if you leave it soaking in water, or in some acid or something it will go bad, but they typically have a very long shelf life
how to do your pads:
1. Remove the brake hose mounting bracket (knuckle mounting part : D).
2. Loosen the guide rod bolt (B) and remove the caliper body (A).
3. Replace pad shim (B), pad retainers (C) and brake pads (B) in the caliper bracket (A).
