Depends upon how much you shoot and how fast you shoot and how hot the ammo. For most of us, a fitted barrel is a life-time investment. If you are shooting action pistol competition, the rifling will wear faster as rapid fire erodes barrels faster than slow fire. The 9x19mm is a high pressure caliber, so it will erode the barrel faster than a low pressure round like 45 ACP. Light weight bullets at screaming velocity will erode the barrel faster than heavy bullets at much lower velocity. If I had to put a number on a 9x19 barrel, based on the "torture tests" done by a gun magazine years ago, about 10,000 rounds if you fire it as fast as you can without stopping. Under normal use, probably 30,000 to 50,000 rounds, but I don't know for certain, I have not shot that much in any one pistol.
