How many miles can an a4 be driven before the
turbo should be replaced?
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.
The general answer is that with proper maintenance it should run forever :-) Regular oil changes with the correct synthetic are critical. If this is a direct injection engine (TFSI) then getting the walnut treatment every 100K miles is important (the direct injection engines build up carbon on the intake runners since there is no fuel in the air to wash them down, power blasting with a soft abrasive, like walnut shells, cleans them up). Having said that, there are certain model year ranges which need turbo replacement by 100K miles. So much of the answer depends on the specific model year and application.