Get a thermometer and stuff it in the center vent with the a/c on coldest
setting, fan speed full, and in recirc mode. 45 degrees F or below should
be fine. If it's a particularly hot day, you should lower the fan speed
and see if the temperature lowers significantly. In these cases, this
volume of very hot air is too much for the evaporator, and the lower speed
will give you an indicator of what the a/c is capable of once the interior
of the car cools.