משנה על בבא קמא 115:1

If wine of terumah was left uncovered, it must be poured out, and there is no need to say this in the case of hullin. Three kinds of liquids are forbidden if they were left uncovered: water, wine and milk, but all other drinks are permitted. How long do they remain uncovered for them to become prohibited? The time it takes the snake to creep out from a place near by and drink.