I was wondering if anyone had a clue on how long a timing belt has lasted before it broke

and how can I check if I need a replacement ( not counting the milage)

A6 quattro 2.8 L 2001

 

You can take the accessory belt off and then remove the front cover to expose the TB. The TB should have no signs of unusual wear, shouldn't have any cracks, stripped teeth, or unraveling fibers. Moreover, the rubber should be still "pliable" and not stiff/dry/brittle.

Here's the 2001 A6 Maintenance Schedule, or here - if you like the .pdf format.

 

a belt can look brand new and still snap ... if in doubt swap it out .. !