I think there are two strategies.
If your trees are still dormant, you don’t want them to endure killing cold, which is probably 15 F or lower. So Strategy 1 is to take them out early – when risk of severe cold has passed but frosts will still occur.
If your trees are growing, you don’t want them to experience any hard frost, which will cause leaf damage. You especially don’t want them to experience any severe cold that would freeze tissues full of sap, which is probably any sustained period 25-28 F or lower. Frozen branches means permanently damaged vascular tissue. So strategy 2 is to take them out when risk of frost has passed.
Growing figs will not be killed by a brief frost, though they will experience leaf damage.
Brief overnight temps <40 F will not send trees back into dormancy, but sustained temps below 40 F will. Leaves will drop and trees will not resume growth for a month.