Time for attainment of steady flow in tube flow

(a) A heavy oil with a kinematic viscosity of 3.45 x 10^-4 m²/s, is at rest in a long vertical tube with a radius of 0.7 cm. The fluid is suddenly allowed to flow from the bottom of the tube by virtue of gravity. After what time will the velocity at the tube center be within 10% of its final value?

(b) What is the result if water at 68^oF is used?