If you are building the amp with discreet components ie bjts then the efficiency depends on the type of amp. class A class B or class AB etc. Theoratically class A has a efficiecy of around 55% and class B around 75% if I remember correctly. class AB is somewhere in between.
If you go for power ICs then you will have to refer to their datasheets.
Of course practically efficiencies will drop below what i mentioned above. hope this helps..
The efficiency also depends on the output level of the amplifier. It isn't at full power all the time, its average power output is probably 1/10th of its max when it is playing music at its loudest without clipping the peaks.
Of course, when it is sitting without an input then its efficiency is zero so its duty-cycle must also be included in an efficiency calculation..