If results with fresh and frozen samples are the same, you can store them for very long time, at least for immunoblotting. now I'm doing immunoblotting with samples being frozen for over one year and everything works perfectly.
Thank you for your answers. The samples are stored in -80 deg C and they were isolated with protein inhibitors so I think it’s a good idea to perform some blots and compare the results to fresh samples.
It depends on the stability of your protein. You definitely have to compare the Western blot results of samples stored at -80C against fresh samples, as indicated earlier. Also I suggest that you avoid multiple freeze-thaw cycles of the stored protein, and instead store in aliquots.