Depends if it is a popular model and your relationship with the AD. You can probably get closer to 10%.
Also depends on the store and who is staffing it at the time. I have bought a few pieces from Selfridges over the years (through its many guises from Wonder room to Bucherer now). Last year I looked at the Zenith Defy El primero 21. All the guys on the shop floor that evening was new and gave me like a 2% discount and refused to budge. I was very polite but they Even told me should take my business elsewhere! So I promptly took my business to another nearby AD who gave a much healthier price!
BW,
Chi Kai
Sent from my iPhone using Tapatalk