Well, from my amateur point of view, but as someone that has done two trackdays at Spa Francorchamps ( Mini Cooper S so not a trackday car):
The sessions there for the particular organization are generally limited at 25 minutes each. This allows the categories to rotate, but supposedly it's for the car to cool down too. I myself felt the car going bad after 20 minutes, tyres too hot, brakes fading etc...Still drivable and enjoyable, but I felt instinctively that the car was degrading permanently. But that is just my impression and I may stand clarified or corrected.
On another hand, there were several Porsche GT3 RS (trackday car) from a club there running with us, with their drivers seemingly professional and dressed like they were Sparco clowns. They were always going out on track after us, and when all the cars got back in the paddocks, they were there already drinking coffee. This triggered my curiosity so I went to discuss with them, and they told me that they got there so often, so they are mostly interested in laptimes at this level, and that the only laps valuable for a lap record were the third, fourth and fifth. Before, the car was too cold and after too warm, so there was no point running them after and degrading tyres or generally the car. So basically they did 13-15 minutes stints.