If you're freezing part or all of blended margaritas, pour them into individual jars or glasses. 1/2 pint (1 cup) mason jars work perfectly for an individual serving size; and, they have lids to make it easy to cover them and put them in the freezer. Leave them in the freezer until they're ready to serve.
The precise answer depends to a large extent on storage conditions — keep opened ready-to-drink margaritas refrigerated and tightly closed. Ready-to-drink margaritas that have been continuously refrigerated will typically keep at best quality for about 4 to 6 months after opening.