I have an a6 ultra, which I’ve done 60,000 miles in. Ad blue top up required every 4,000 miles or so which is a pain and every other month for me.
The most annoying thing is that the tank is 17L and it comes in 10L refills and it’s hard to assess when it’s full.
I got the car two years ago and at the time ad blue was very expensive, about £38 for 10L. That has come right down now to about £17 for 10L.
Audi dealers are starting to get filling machines too so getting a full 17L fill is possible, and so the range goes up a bit.
Still, it was a major reason I decided to buy a petrol (S4) replacement. Ad blue is a pain for which there is no completely satisfactory outcome yet.