A long pryoceram rod with a diameter of 80mm is clad with a very thin metallic tube for protection. The bonding between the rod and the tube has a thermal contact resistance per unit length of 0.1 mK/W. The rod is initially at a uniform temperature pf 900K inside a furnace and is removed to be exposed to the air ar 300K and a convection coefficient of 200 W/m^2K. How long will it take for the centerline to reach 600K?