What is the normal fan speed on Mac computers
There is no "normal" speed for a fan in a computer.  The fan spins at a speed necessary to keep the CPU temp in the "normal operating range."1

For your particular Mac, the  maximum operating temperature is about 82°C (180°F).  However, keep in mind that this does not factor in other components like heat from drives, adapters, etc. which may also go into calculating when a fan is engaged.



1 See section 5 in Intel® Xeon® Processor E5-2400 Product Family Thermal/Mechanical Design Guide May 2012