Poiseuille's law as stated here gives the volue time rate of lamnar flow of an incompressible fluid through a pipe. R is the radius of the pipe, l is its length, Δp is the pressure drop across the length of the pipe, and η is the coefficient of viscosity of the fluid.
dV/dt= πR4Δp/(8ηl)
Calculate the time to drain water at 20C from a vertical tank 1.7 m in radius from a depth of 13 m to 1.6 m through a 1 cm in radius pipe 12 m long connected vertically at the bottom of the tank. Take η = 1 cP where 1P = 1 g/(cm s)