It's really not a mileage thing, more of a pressure output, when the boost pressures start to drop from what they normally are, then it's time for a replacement. Or if it starts to blow oil into either side of the turbo.
The turbos can/will last over 100k miles with proper maintenance and scheduled oil changes. Also, changing the oil feed lines every 70k miles will give it a lot more life.
Search Audi A4 Questions
Audi A4 Experts
Related Models For Sale
Used Cars For Sale