As fries cook in the oven or fry in oil, the heat draws out moisture. The starch in the potatoes prevents moisture from escaping, which is why soaking them creates crispy fries.
Soak your cut potatoes for a minimum of 15 minutes. A longer soak time removes more starch, so if you can prep the fries beforehand, try to soak them for a few hours.
When making thicker-cut fries, the potatoes should soak for up to 24 hours, or 12 for thin, shoestring fries. After too long, the potatoes absorb water, resulting in a mushy mess.
Store your spuds in your refrigerator to keep the water cold while soaking overnight. Any added heat might activate the starch and lead to uneven cooking or unpleasant textures.