I said 11-15 but it's not that simple really.
If I'm driving a car I like and I'm used to, I'll just keep driving until I feel I can't get more out of the car. At that point I'll judge based on my pace if I need to switch cars or not.
If I'm not sure about if a car suits the track, but it's still a car I'm comfortable with, I'll do 11-15 to evaluate the combo and then switch if I'm not happy.
If I'm trying a car out that I'm not used to, I sometimes call it quits after only 5 laps if it's just not working for me. I don't need to punish myself more if I can't keep control over a car.