Citrus trees will die in 10°F weather in 10 hours and 20°F weather in 20 hours.
They will survive in 30°- 40°F weather, but they will not be happy. They will drop many leaves but should recover.
We recommend bringing your tree indoors or into a garage when the temperature is consistently below 40°F.
Exposing your tree to very cold temperatures is the quickest way to kill it.