$${\Delta t = \frac{4 \pi (3,5\, \mathrm{m})^2(3,5 \times 10^{-19}\, \mathrm{J})}{(1,5\, \mathrm{W})(7,85 \times 10^{-21}\, \mathrm{m^2})} \\ = 4850\, \mathrm{s} \approx \underline{\underline{1,3\, \mathrm{h}}}}$$