How many degrees should home air conditioning be at the vents?

measuring how effective your air conditioning is at the supply air diffusers will not be an accurate gauge of it's function. The temperature measurements should be taken just above and below the evaporator coil. The return temperature (below the coil, if not possible, then upstream of the blower) should be 15-20 degrees Fahrenheit higher then the supply temperature. Keep in mind that this temperature difference is affected by the blower speed as much as it is by the function of the air conditioning system. Therefore it can only indicate that something is not functioning correctly, but will not troubleshoot which part of the system is malfunctioning.