A wet solid of $100\:kg$ is dried from a moisture content of $40\:wt\%$ to $10\:wt\%$. The critical moisture content is $15\:wt\%$ and the equilibrium moisture content is negligible. All moisture contents are on dry basis. The falling rate is considered to be linear. It takes $5$ hours to dry the material in the constant rate period. The duration (in hours) of the falling rate period is ________________