Hi Patrick
The pad replacement period is calculated from when it is reset (if it is done properly) and then the miles count down.
The wear rate obviously depends on how the car is driven so the replacement period is only an estimate but I would err on the side of caution and look to get them replaced as soon as possible, along with the discs (if required).
I also would recommend a good independent repairer as should be cheaper than the dealership. Ensure they reset the interval on the dash and it should not affect the warranty as long as parts of equal quality have been fitted inline with correct fitting procedures.
However, spend as much as you can afford on decent pads as I find I am replacing cheap pads at an alarming rate due to poor performance so as the old saying goes 'you get what you pay for'
Let us know how you get on with it all
Cheers, Trevor