If the amp draws 5.5 amps at 120 volts then 660 watts is the most that can be converted to heat (energy is always conserved). That can mean, for example, you will need an additional 2,250 btu/hr for a room air conditioner just to remove the heat from the amplifier.
To get an exact number is tougher to calculate. The amp at idle will draw the bias current only, and that can be a value which is a fraction of the 40W plate dissipation. When the signal is going through the amp, the current is fluctuating and rarely goes to max plate current, so the overall heat dissipation can only be determined by monitoring the current draw over a given period of time (say, an hour). Then you can get a more precise number. If all you want to do is size an air conditioner then just use the rated amp draw, or use the nominal amperage of the mains fuse.
