I just recently changed my timing belt in my 04 A4 at 112,000 miles. When I took the old one out it was in great shape. It probably would of lasted another 50,000 miles easy but you dont want to risk it. If your timing belt snaps while driving your engine is going to see some serious damage and so is your wallet. Its recommended to change a timing belt at 100,000 miles. its also a good time to replace the water pump, serpentine belt, belt tensioner, and thermostat. At an Audi dealer I spent $520 for all of these parts (you can find cheaper online but for parts that make your engine run I would get genuine Audi parts) I did the work myself. You can find forums on how to do it online but if you dont know too much about working on cars you might want to ask a friend. Its a lot cheaper than spending an extra $700 for labor.