-25 degrees Celsus is a significantly LOW temperature. I would expect the film to last ... oh ... something like one hunrded years, or so.
What is involved is "Chemical kinetics" ... and the Arrhenius equation would seem to be appropriate in calculating a more coherent shelf life than I described above.
A useful generalization: (In) "Many reactions which occur near room temperature, an increase of 10 degrees Celsius approximately doubles the rate of the reaction." So, in reducing the storage temperature, the shelf life is increased. Given 20 degrees Celsius as usual room temperature, shelf life will be ~ doubled at 10 degrees C; doubled again at 0 deg. C; doubled again at -10, again at -20, yet more at -25. If the usual shlf life is five years, we are near the 100 year mark at -25.
I've worked with the Arrhenius equation, and related, in Reliability and Environmental testing. We had a nice, neat little computer program - put the temperature, and other significant factors in, and projected "Life" information would come out. Unfortunately, I don't have access to the program any more ... so, possibly someone else here (Listening, PE?) would care to be more accurate.
You might want to try Arrhenius/ Arrhenius equation through Wikipedia.