Answer
2700 years.
Work Step by Step
Decay constant $k=\frac{0.693}{t_{1/2}}=\frac{0.693}{5.73\times10^{3}\,y}=1.2094\times10^{-4}\,y^{-1}$
Recall that $\ln(\frac{A_{0}}{A})=kt$ where $A_{0}$ is the initial amount and $A$ is the amount after time $t$.
$\implies \ln(\frac{100}{72})=0.3285=1.2094\times10^{-4}\,y^{-1}(t)$
$\implies t=\frac{0.3285}{1.2094\times10^{-4}\,y^{-1}}=2700\,y$
