About twenty-four hours, and only if they are completely submerged in cold
water. Left dry on a plate, peeled potatoes go grey within the hour — that part is
oxidation rather than spoilage, but nobody wants to cook it.
Water works because it blocks the air, which is the whole mechanism. Peel and cut them the
way you intend to cook them, put them in a bowl, cover completely — anything sticking out will
darken — and refrigerate. Room-temperature water does not buy the same time.
A squeeze of lemon juice helps and is not essential.
What the method costs you is starch. Water leaches it, and potatoes soaked much beyond a
day turn watery when boiled and make a poor mash. So prepping a day ahead is genuinely useful;
prepping two days ahead is a false economy. For roasting or chips the loss matters less —
some recipes soak deliberately to remove surface starch and get a crisper result.
Raw potato does not freeze, peeled or otherwise: it turns sweet and mushy for reasons
covered in freezing
potatoes. If you want potato waiting in the freezer, blanch or part-fry it first.
And if the point of peeling ahead is to save time, cooking ahead is the better plan.
Boiled potatoes keep three to four days refrigerated and reheat, fry or go into salad
with none of the discolouration problem — which makes them a more useful thing to have
in the fridge than a bowl of raw ones under water.
See also:
How long do cut vegetables last in the fridge ·
Why deruny turn grey ·
How long does potato salad last in the fridge ·
Why potatoes darken after boiling