Excusing any details like large caliber and extreme rates of fire it is common to answer this question by saying a few thousand bullets. That answer is fairly accurate. However there is another way to think about it that really puts it into perspective. Would it shock you to learn that average barrel life is under six seconds? Gone in Six Seconds — The Shocking Truth of Barrel Life « Daily Bulletin

Maybe if you a bench rest shooter... I dont seem to have any problems with my ol' mil-surps and im sure they have shot more than 3,000 rounds...

That 6 second rule for barrel life does vary a few fractions of a second depending on the caliber and velocity of the bullet. 243 Win. is a total barrel burner and 300 Win. Mag. is a throat burner as an example. The barrel of that 300 Win. Mag. I had built last year for 1000 yard target rifle lasted only 1132 rounds which is normal life span for how I used it. I'd be curious of how many seconds that 1132 rounds equates to.

Assuming an average muzzle velocity in excess of 2000 fps along a 24" barrel the bullet is in the barrel 1 one thousandeth (.001) of a second. Your total bullet-in-barrel time is about 1.1 seconds; (1,100 x .001)

Another way to think of this. Cost per hour of work. At $300 a barrel and assuming were getting a full 6 seconds of useful work out of the barrel we need to replace 10 per minute and 60 minutes in an hour. $300 X 10 X 60 = $180,000 per hour! How much do you get paid per hour? About 2000 hours is the normal amount of time a person works in a year. To hire a barrel for a year would cost $360,000,000.

Well a union barrel would cost 30,000$ each work for .04 seconds each 1000 rounds and protest governer scott walker of wisconsin the rest of the time

ok...here goes... 3080 feet = 36,960 inches 36,960 inches = 1,320 28" barrels bullet is in barrel .00075 seconds 1,132 rounds x .00075 seconds per round = .85 seconds bullet-in-barrel time.

Dave Petzel wrote a good article about barrel life in an old issue of Field and Stream. The central point was it depended on how hot a load you were putting through the barrel and whether you allowed the barrel to cool between shots. I'll go dig out the issue and give you the year and month.