Middleton Tompkins, noted Long Range & Palma Rifle Champion, says the life of a barrel is about 3 seconds.
After that you will start seeing deteriation of groups.
Sooo, what you have to do, is figure out how long your bullet stays in the barrel, then go from there.
For a 24 inch barrel that would be Time in barrel = (V1+V2) / 2
V1 would be '0", V2 would be MV. (Lets say 2600 FPS)
So lets say 0 + 2600 / 2 = 1300
So now we divide 1300 by 2 (24" is 2 feet) = 650
So 650 rounds would spend 1 second in the barrel.
To have a bullet in the barrel would take 1950 rounds.
Again thats for target quality. I don't know how he accounts for faster bullets causing more wear. I think its because he shot 155 grn Palma bullets at 2900 fps.
Personally I don't shoot that well.
You can tell when your barrels going south by the X count.
As a side note: We all know copper is softer then steel so it stands to reason a steel jacketed bullet would do more damage to the barrel the copper, and lead being softer then both, shoot lead bullets would extend the life of your barrel.
Barrels first start going at the throat. That's because of the burning gas eating away at the throat. Choosing the proper powder for a given round will reduce throat wair.
Also, on my target rifles I start with a longer barrel. As the throat starts going south, I can take the barrel off, set it back, tread and re-chamber it and extend the life of the barrel.
Take into account, more barrels are destroyed by improper cleaning then wear.
Especially on military rifles where GIs use a jointed steel cleaning rod cleaning from the muzzle.