To be precise, they apply ((listed damage+2)/3), rounded down, to the squares they affect every 1280 ticks. Since a combat round (aka: turn) is 7500 ticks, a cloud can usually apply very slightly less than 6 times per turn. A cloud will trigger four times per point of Lingering listed on the spell, so each point of Lingering can also be thought of as making the cloud last on field for 68% of a round.