Preferably best to freeze immediately after expressed. But in your case, do you know EBM can be stored in the fridge for up to 3 days.
Under most conditions, breast milk stays fresh at room temperature (60 to 85 degrees Fahrenheit) for three to four hours. But expressed milk that has little bacteria can last for six to eight hours if room temperature remains on the cooler side.
Frozen breast milk stays fresh for two weeks in a refrigerator that has a single door to the freezer and refrigerator compartments.
In a refrigerator that has a separate door to the freezer, frozen breast milk lasts for 3 to 6 months.
Otherwise to ensure less bacteria and more nutrients are stored. Wash bottle and hands before pumping. Pump and freeze immediately.
Just my 2 cents. :)