A pure Silicon wafer is doped with Boron by exposing it to $B_2O_3$ vapour at an elevated temperature. It takes 1000 seconds to reach a Boron concentration of $10^{20}$ atoms $m^{-3}$ at a depth of $1 \, \mu m$. The time taken to reach the same concentration of Boron at a depth of $2 \, \mu m$ is (in seconds): ................ (rounded off to nearest integer).