A fiberboard sheet $\left ( 1.5\:m\times 2.0\:m\times 15\:mm \right )$ is being dried by suspending it horizontally in a current of hot, dry air. The edges are insulted so that drying takes place only from the top and bottom surfaces. The wet sheet weighing $16\:kg$ with initial moisture content of $60\%$ loses moisture at a constant rate of $1.25\times 10^{-5}$ $kg\:m^{-2}\:s^{-1}$ until the moisture content falls to $30\%$. All moisture contents are on dry basis. The time required for drying during constant rate period (in hour) is ________________ (rounded off to third decimal place).