Two cylindrical shafts $A$ and $B$ at the same initial temperature are simultaneously placed in a furnace. The surfaces of the shafts remain at the furnace gas temperature at all times after they are introduced into the furnace. The temperature variation in the axial direction of the shafts can be assumed to be negligible. The data related to shafts $A$ and $B$ is given in the following Table.
$$\begin{array}{|l|l|l|} \hline \text{Quantity} & \text{Shaft A} & \text{Shaft B} \\ \hline \text{Diameter (m)} & 0.4 & 0.1 \\ \hline \text{Thermal conductivity (W/m-K)} & 40 & 20 \\ \hline \text{Volumetric heat capacity (J/m^3-Km)} & 2 \times 10^6 & 2 \times 10^7 \\ \hline \end{array}$$
The temperature at the centerline of the shaft A reaches $400^ \circ C$ after two hours. The time required (in hours) for the centerline of the shaft B to attain the temperature of $400^ \circ C$ is _______ .