The short answer for a Plymouth Rock in an emergency
Seven days is the minimum emergency store for a Plymouth Rock hen, and fourteen days is the wiser target where power cuts, frozen pipes, or blocked roads are realistic. A Plymouth Rock is a generously built dual-purpose hen, bigger than a slight laying hybrid, and her daily feed portion matches that frame: she eats a steady measure of layer feed each day and drinks roughly half a litre of water in mild weather, doubling that in heat. Multiply her real daily amounts by your chosen number of days, then add a cushion, because some feed is always scratched into the bedding and some water is always spilled, fouled, or frozen at exactly the wrong moment.
What belongs in the Plymouth Rock's feed store
Store the layer feed the Plymouth Rock already eats, kept in a sealed metal or heavy plastic bin that rodents cannot chew through and damp cannot reach. Alongside the feed, store the two things a yard normally supplies for free and a coop-bound emergency does not: grit, which she needs in her gizzard to grind whole grains and pellets down, and a calcium source such as oyster shell for the hens laying through the disruption. Both keep almost indefinitely in dry containers, so there is no reason for them to be the missing item. If your flock eats scraps and range pickings as part of the ration, size the stored feed for the full diet anyway, because a hen shut in the run during storm damage or a predator scare eats entirely from the bin.
Planning water a laying hen can actually reach
Water decides how a Plymouth Rock copes, because a laying hen short on water stops laying quickly and declines faster in heat than a hen short on feed. Store drinking water in covered containers kept out of direct sun, and plan for daily drinker changes rather than one large fill, since hens foul open water with bedding and droppings within hours. Keep a spare drinker in the store itself: the common emergency is not an empty reserve but a cracked or frozen drinker with good water standing useless beside it. In cold snaps, plan how drinkers will be swapped for unfrozen ones each morning; in heat, plan a second drinker in the shade of the run so the whole flock is not queuing at one spout.
Rotation and checks that keep the store honest
An emergency store for a Plymouth Rock stays honest only if it is used in ordinary times: feed from the stored bin week to week and refill it with fresh stock, so no sack sits long enough to go stale or attract weevils. Refresh stored water on a regular cycle and write the fill date on each container, and check drinkers, lids, and bin seals whenever you do a coop clean-out. Recount the store whenever the flock changes size, because a reserve built for six hens is a three-day reserve for twelve. Note the daily portions on the bin lid itself, so the amounts survive even if the person who set the store up is away when it is opened.
If someone else has to run the coop from your store
Emergencies often mean a neighbour, relative, or house-sitter feeding the Plymouth Rock instead of you, and the store should explain itself: how much feed per day, where the grit and shell sit, how often drinkers get changed, and where the spare drinker is kept. Leave those notes at the coop, not in a drawer indoors. A short trial run, with the helper doing one full feed-and-water round while you are home, exposes every gap in the written plan while it can still be fixed calmly. KinsLe Family Sharing keeps the hen's daily amounts, store locations, and helper notes on her shared profile, so whoever opens the coop works from the same instructions you wrote.