I'd suggest a month, too, since that's roughly how long it takes between ferry arrivals. Maybe a little more.

( if something were to happen to the shipment, then they'd hopefully have enough to pull them through the month without the same concerns and shortages that have cropped up in the past. )

That's true. When the rats attacked our food supply, we had little defense for it. It might be wise to have more than one location, and somewhere secure.