Mileage will be the largest contributor, but age, weather, and typical commute will take its toll. Give you an example, my wife’s i3 REx has regenerative braking, so the discs or pads are hardly used. That said we live in Scotland, so the discs take a beating with road salt. I’ve changed them twice (2015 build) with Textar (BMW OE). Car on 90k miles. Good opportunity to strip down brake sliders, check rubber dust boots, ceramic paste on pad backs, etc
On a 2020 A6 it would be worth a brake strip down even if the pads and discs look fine. You’ll need VCDS for rear electro mechanical handbrake. Will be due brake fluid change too. If discs have a lip, then they will be due. Any scoring too.
Front brake carrier bolts are known to seize. Not unique to A6, on A4 platform too due to oxide corrosion.