"During storage offrozen dust/filter extracts for 3–4 months at201C, a loss of approximately 40% of measurable a-amylase was observed, which could be partially prevented by addition of 0.1%casein to the medium directly after extraction." Here is the link:
Article Optimized methods for fungal ?-amylase airborne exposure ass...
Thus, I suppose that you should not store your PBS-amylase solution in refrigerator longer than 2-3 days.
Best of luck in your research!
Yours sincerely,
M. Sc. Vadym Chibrikov
Department of Microstructure and Mechanics of Biomaterials
Institute of Agrophysics, Polish Academy of Sciences