The answer depends on what kind of snorkel you have. Full face snorkels with no tube or external oxygen supply can quickly run out of air. You shouldn't spend more than 1-2 minutes underwater with this kind of mask. Traditional snorkel masks with tubes above the surface can be used indefinitely, provided the user isn't feeling faint or dizzy. Some divers choose to hold their breath, dive deeper, then return to the surface for a short period to recycle their air.