When should a disc brake be replaced?
There is no way you can accurately assign a distance to disk brake replacement.  Someone commuting in a stop and go method in a wet climate like the pacific northwest will get much less life out of their setups than someone living in a drier/cleaner climate and a more continuous commute.

Anyone can determine the relative life left in their setup by looking at their brake pads.  The pads are the wear point for a set of disc brakes and DO need to be replaced with normal use.  Almost all other aspects of the system are likely to last near indefinitely with normal use (except perhaps the cables on a cable actuated system).  

For your part, you would do well to check you brake pads IMMEDIATELY.  If you wear all the way through the pads and do not replace them you have a high chance of ruining your piston as well as possibly your rotor.  Ruining the piston will essentially result in needing to replace the entire caliper and/or unit.  Hopefully the horrible noises you are hearing are just the pad backing tearing against your rotor and you haven't damaged your pistons yet.  You may get away with just new pads and/or rotors.