Basically never. On very very rare occassions, like when it's 100+ out I will reach back and touch my amp and see how hot it is. If it's hot to the touch I turn the fan on, If not I leave it. I have faith in Knu Konceptz, My woofer builder (myself), the amps, and my Installer (myself). I agree with what Alpine said when you do an install for a customer you can't check it 4-5 times a week so do every install as if it was for the dumbest crackhead out there who doesn't check squat and go on.