Two radioactive materials $R_1$ and $R_2$ have decay constants $6\lambda$ and $\lambda$, respectively. The half life of $R_2$ is $1.4 \times 10^{17}\,\text{s}$. Initially they contain some number of nuclei. The time at which the ratio of the remaining nuclei of $R_2$ to that of $R_1$ will be $e$ is (Let $\ln 2 = 0.7$)