So many variables, like class of amp, volume of air circulation, etc. I do not believe your question can be answered as stated.
Electronics have an acceptable temperature operating range. Environmental conditions (ambient air temperatures) fluctuate. Each installation requires the amplifier to be operated within the acceptable operating temperature range. If environmental conditions (like mounting orientation) cause the amp's temperature to rise above the acceptable range, cooling fans or other means must be used to ameliorate this condition.
Hope that helps.
'89 BMW 325iX | Focal 130CVX Polyglass
Trying to figure out CD43 head unit and ESX 120.4/475.1 Lukebox install