I am experiencing very similar.
S4 Avant delivered two weeks ago - variable servicing with 18,000 miles remaining to oil change.
I've now done 500 miles (mainly consisting of 30 mile round trips to work, mix of A road and motorway), and that has plummeted to 14,300 remaining to oil change. Last night, drove 5 miles and it reduced by 300 in one go!
I will be into oil change territory by 3000 miles.
In years to come, I think this "condition based servicing" will become known as one of those scams similar to the emissions scandal. It is hard to believe that a few short journeys reduces the oil quality that quickly.
Thankfully, my lease is fully maintained, so not an issue, I'll just chuck the car in for an oil service every 3000 miles - inconvenient though!