the manufacturer recommends repalcement at 52,500 miles for severe service and 105,000 miles for normal service.
Here's a directory link for you:
The manual says 105k but you should do it MUCH sooner. If it breaks it goes from 1000 to 3000k repair. Here is a link to maintenance schedule for the A4. https://wiki.bentleypublishers.com/display/tech/Audi+-+2001+Maintenance+Schedules;jsessionid=3B9D96D96E9C2D4E64C1E0CB98A3C4A2