Google the Chuck Taylor Glock 100,000 test. He never mentions replacing the extractor. If yours is showing signs of wear or damage, then I would talk to the folks at Glock. None of my Glock extractors have any real signs of wear. I could see replacing the extractor spring eventually though.
The rifling in a glock is not sharp even coming from the factory because they use polygonal rifling that is forged into the barrel instead of cut rifling. This increases the life of the barrel and makes cleaning easier.