Just got rid of my Peugeot 208 Gti which I owned from new and had 146k on the clock and was still giving the same MPG as when I bought it and running sweet. Moral of the story - I now have an A4 avant with 46k miles on the clock - looking to change oil every 6k miles and as it's much better build quality than the Peugeot, I hope to keep it for many years and 100k should be easily achieveable if not 150k..
Keep you car regularly serviced and it should keep on going.. good luck.. and they are fab cars.. I had owned an A5 3.0 TDI Quattro sport before and regret selling that, so I'm sold on Audi