Breech plug live varies. Like Mark said, I too have a Wolverine that I have shot more rounds then I care to think, with #11 caps for years and it shows little to no wear. But on the other hand, I have a Knight Disc that uses the orange primer jackets, and it has worn out with only around 500 rounds. That's because the primer ignites before the breech plug.
Now on some of the other rifles I can not judge. I have a CVA Staghorn Magnum that has a thousand rounds through it, and shows no wear. I have a Black Diamond XR that is on its second one.
Keep your eye on the threads. Keep your eye on the vent hole, front and back. You will be able to spot wear. At that time, replace it.