My bank told me the reason is the feds are capacity limited on coin decontamination. Bills they just burn and print more, but coins cost too much to re-smelt.
2 weeks if you are really worried.
For most supply chains, adding a week of inventory would mean bringing on additional warehousing space and filling everything they have to the gills. I'm familiar with a factory who has a few hours of inventory on-site, and no more than ~3 days in the accompanying warehouse. They'd need about 2 more equal size warehouses to add a week of inventory.