Just got mine done at Audi Sandton and as I aways leave my car there @7:00am it wasn't an issue the car was ready cleaned and polished by 17:00 same day.
Now if you are a new unfamiliar face to the service advisor they will tell you 2 days (or if they are overbooked and busy like Sandton) but with a little persistance they will accomodate you.
Courtesy cars from busy dealerships is very rare (unless you both your last S6 RS6 from them) the only courtesy cars I have driven were VW Beetle (new one) and Golf Chicco ;-( I know they never have anything on hand...unless you book the courtesy car 2 weeks in advance with the pretext that your wife is due to give birth on the chosen day

Try it might work