Answer
$\Delta t =3.21 years$
Work Step by Step
$E=\ 350/year\div\ 0.11/kWh=3182\ kWh/year$
$Q=0.95\times E=3023\ kWh/year$ to the water
$COP_{HP}=Q/W=3.3$
$W=916\ kWh/year$
$C_{HP}=916\ kWh/year \times \ 0.11/kWh=\ 100.8/year$
$C_s=350-100.8=\ 249.2/year$
$\Delta t = \ 800/C_s=3.21 years$
