I have 130k on my GTA and I'm trying to find the source of a slight engine knock. Should I replace the ignition coil? I believe it's original..

 

I don't see how a coil is causing your knock, look elsewhere.

As for how long they last. Consider they have no moving parts and are basically a lot of loops of wire, theoretically they last forever.
Occasionally after enough heat/cold cycles, vibrations, and maybe some corrosion a wire breaks in one of the loops leaving you with a dead coil.