As usual, the answer is depends... some varieties have been bred to have more cold tolerance than others, wet cold vs dry cold makes a difference, etc. Also are you just concerned about the plant surviving or the fruit? If temps get below 50, it is likely to drop blossoms instead of setting more fruit. But it won't drop fruit that are already set. If the plants are protected and have been getting used to the cold gradually (a sudden cold snap when it's been warm is worse than gradually cooling temps), the plant should survive temps down into the upper 30's.
http://www.helpfulgardener.com/forum/viewtopic.php?t=31381
I'm having a problem with it being too hot in my grow room... The past two days here in SE Wa have been pretty hot, and when we turned on the lights last night, it was ~90, and it was about the same all night long, even when I turned em off at 9 this morning...
https://www.growery.org/forums/showflat.php/Number/478530
Along with the CCA (cold cranking amps) the  consumer ought to also consider the AH rating (know as the  AmpHour Rating). This is the specification that tells you how long  you can accidently leave the lights on or an accessory on before  the battery will be drained dead.
http://www.answers.com/Q/What_happens_if_a_plant_is_too_cold
When the Body Gets Too Cold Once temperature goes below your bt(body temperaturea), your body looks for ways to warm you up- shivering, goosebumps. If your body is exposed to …cold temperatures too long and loses too much body heat, you freeze to death.
http://www.answers.com/Q/What_happens_if_a_plant_is_too_cold
What happens when you have too many Cold Cranking Amps? There is no such thing as too many cold cranking amps! The battery  does not push the CCA (cold cranking amperage) but merely has it  available to the starter to draw from t…he battery as the starter  needs it.
http://www.answers.com/Q/What_happens_if_a_plant_is_too_cold
Temperatures below 70 degrees F are too cold for most Cannabis plants. Ideal temperature for daytime should be 70-80 degrees F. Night temperatures should be kept over 60 degre…es F to prevent stress, stimulate hormones and reduce stem elongation.
http://www.answers.com/Q/What_happens_if_a_plant_is_too_cold
I'm in New England and it's already pretty cold here at night. I was wondering how cold can it get before marijuana starts to get damaged or whatever? I heard someone say they can't take anything under 40F but I don't know. I thought that as long as it wasn't freezing it shouldn't hurt the plants.
https://www.growery.org/forums/showflat.php/Number/478530
Many plants can take a short mild freeze (30-32F). But if the temp stays below freezing for more than a few hours, it could kill the plants. Pay close attention to the weather forecasts and apply the information to microclimate where your plants are growing. Be ready to harvest if weather dictates.
https://www.growery.org/forums/showflat.php/Number/478530
The more undamaged foliage there is, the more that foliage can feed the roots, building up the plant for a bigger, better display next year. Continued in Part 2, where I illustrate how appearances can be deceiving, what happens to trees and shrubs, and a few plants unfazed by the dramatic temperature drop.
http://www.coldclimategardening.com/2009/05/27/what-happens-to-plants-after-an-untimely-freeze-part-1/
Part of what makes my cold climate garden cold is the fact that we live in the bottom of a valley, and cold air flows downhill and settles all around us. So however cold the weatherman predicts it will get, it’s usually colder here. Often, ten degrees colder. For example, on May 12th, when a light frost was predicted, the low temp in the morning was 28F(-2C). The following morning, it was 28.5F (-1.9C). I call those two hard frosts. And on the 19th, it was 25.5F(-3.6C).
http://www.coldclimategardening.com/2009/05/27/what-happens-to-plants-after-an-untimely-freeze-part-1/
what happens when a plant gets too cold
When a plant gets too cold then it can be killed. If the temp stays below freezing for more than a few hours, it could kill the plants.