Normal freezing condition (4oC) you can keep for 7 days for best use & for15 days maximum, 7 day is best. You cann't keep the extract in icing condition cause your extract may increase in volume because of icing and melting. It may reduce its potency. Always use fresh extract to attain best result in antibiotic assay.
I think plant extracts extracted with organic solvents, not affected by long-term storage under the freezing condition, especially if they placed under (-18oC)