how long should the turbo in an audi a4 sedan last?

Check with the owners over on Audiworld.com and Audizine.com - they would know how long they last and what would be a good replacement.

turbo replacment is the most simple thing in the world you can find in youtube how to replace it guides, and second of all length of a turbo is determine by the way you drive, that means if you will start the car and hit the gas and won't let the engine heat first it won't last month. so temperature of the engine is a most thing to remember. only when the car reaches its normal temp' then u can go wild.. ;) have fun!