Answer
17200 years.
Work Step by Step
Present activity $N=\frac{1}{8}N_{0}$ where $N_{0}$ is the original activity.
Rate constant $k=\frac{0.693}{t_{1/2}}=\frac{0.693}{5730\,y}=1.21\times10^{-4}\,y^{-1}$
$\ln(\frac{N_{0}}{N})=kt$
$\implies \ln(\frac{N_{0}}{\frac{1}{8}N_{0}})=2.08=1.21\times10^{-4}\,y^{-1} \times t$
Or $t= \frac{2.08}{1.21\times10^{-4}\,y^{-1} }=17200\,y$