We do not recommend to store the solution with PBS at -20ºC. By using PBS as a buffer and storing at -20ºC you will create phosphate acid and phosphate acid can damage the proteins. If you are going to use PBS buffer, you can store it in a refrigerator at 2-8ºC. The time you can store it depends on the quality of your PBS and how sterile you are working. If you really want to freeze the solution we recommend to use 0.9% NaCl. In general, we recommend to freeze-thaw your material as little as possible. Find out more here.