Vent liners should be replaced when they open to .036 or more. We sell gage pins if you don't want to do the stick something like a torch tip cleaner through and measure it thing...
The older vent liners (black flat heads) were good for 40-50 shots. Then Lehigh made them out of what they said was "mil spec" alloy: might get 100 shot out of them if they were heat treated. They heat treated the ones we sold, and didn't heat treat the ones that got soldered into knight breech plugs.
Now we get them from Lehigh in 17-4 stainless, heat treated and carbonited. They are lasting much longer... like 100-150 shots. I have one customer saying he has 300 shots on one of our stainless and it still is under .036... I haven't proven this out myself yet. Just haven't been able to shoot that much... and don't want to burn up the Black Horn 209 I have.
For me, as a hunter, I'd get a Savage BP with the vent liner. You can buy a lot of the stainless vent liners from us and a gage pin for what you'd save over the carbide busing BP.... enough that most folks will never shoot them all out.
But I'm biased in that I do sell the vent liners.. but they just make more sense to me.