72 hours. Suppose that the water injection per hour of each water injection pipe is X, the water discharge per hour of each drainage pipe is Y, and the total volume of the pool is Z. The time required for the problem is T. Then it can be expressed as: 8 (12x-y) = z, 24 (9x-y) = z, t (8x-y) = z. According to the first two formulas, y = 60.