The ideal is to prevent an attack, not prolong one, but sometimes we aren’t in a position to get to a warm place soon. There are medications called calcium channel blockers that help to keep the blood vessels open (dilated) and prevent, or, lessen the severity of an attack. If you haven’t done so already, maybe consult with your doctor about trying these.
There is no time frame on the amount of cold exposure one can experience before tissue damage sets in. Basically during an attack, your vessels constrict, the tissue is deprived of blood and oxygen and damage can occur. I wouldn’t suggest putting this to a test.