There are too many variables to accurately determine the temperature rating.  When was the bag manufactured?  Was the bag stored in a stuff sack or hung upside down?  Was it stored in a cool, dry place like a closet on the main floor, or in a hot garage or storage shed?  If it is filled with down, what is the fill power and weight?  If it is filled with synthetic insulation, what generation of synthetic?  Hollofil has a dozen different generations of that particular type of insulation.  What is the fill weight of the synthetic insulation?  If you are not the original owner and cannot definitely answer all of these questions, it becomes exceedingly difficult to give you an accurate temperature rating.