This is a tricky one...might put my RS6 on Consignment....

[nubcake]

Keep in mind, that you're putting those rare turbo 5v heads at risk.
5y/35k is a sane enough interval. You can delay the replacement, if you have to - but read above.
I wouldn't push it past 50k miles. Age - less critical in my book, but be sure to regularly check for any signs of drying out (cracks etc).

[GreggPDX]

My understanding is the interval might be related to the bearings. Those belts should not wear significantly faster than other cars, but the guide rollers are stupidly small. You could probably run the belt a long time if you could inspect the rollers regularly, but by the time you tear it all down to inspect, you might as well replace them and the belt.