There is a lot of difference of opinion and instruction from manufacturers on this.
To err on the side of safety, 24 hours above 60.
I personally take into account the previous nights temperatures, what side of the house the unit is on (sun or shade), what the current temperature is, and whether I'm feeling lucky
.
But Rick said it right, you aren't going to prove much about the performance when it is close to the limit.
It just is not worth the risk.
Most of the time there is other problem with the system that I recommend service anyway, and I am not bashful about telling them it is too cold to test the unit.
Bruce is right about the 12 hour period of power is for crankcase heaters. Not all units have them, but I am seeing lots of stickers on the units lately.
If the units have crank case heaters, the heat keeps the liquid refrigerant from migration to the compressor pot.