How long do you boil a potatoe

Beyond the guidance above - "until they're done". Test by inserting the tip of a knife. If the potato sticks to the knife, it's not cooked. If it falls off after 5-6 seconds resistance it's par-boiled (almost there), if it falls away after a second or two you're almost perfect, and if it doesn't stick at all it's well done to overcooked.

Ah, thanks.

Merrill, why do you start the potatoes in cold water? I know to do this but have never thought to ask why until just now.

If you're cooking small potatoes, or if you cut larger potatoes into 1-2-inch chunks, they should take 15-20 minutes once you've brought the water to a simmer. Start with the potatoes in cold water, and don't forget to salt the water!