I do not get more than 2 years out of brake discs even when doing 28k per year. I could push it to 3 but I noticed the degradation in brake performance. My discs tend to suffer from blackening even when the car is used 6 or 7 days per week throughout the year.
I know I am worse than most and I've worked out the reason for that. It is because I use the brakes less than most and don't use light check braking for extended periods which seems to be the norm these days. Because I don't use the brakes for extended periods, the discs don't have the same cleaning action which causes corrosion to be able to start to build up in the form of black pitting.
As said, this only becomes noticeable when braking at higher speeds and I don't let my cars' braking performance degrade to the point that most people do. Years ago, I nearly witnessed another car take out a group of people which was obviously down to poor braking performance. It's just one of these things which has stayed with me.
I point this out because the thread is really about braking performance and people who look at upgrade brakes tend not to have such an ambivalent/uneducated attitude to car and brake maintenance as the majority.