A $600\ \text{W}$ immersion heater is placed among $0.40\ \text{kg}$ of crushed ice at $-10\ ^\circ\text{C}$ in a well-insulated flask, and switched on. All of its heat goes into the ice and the water it melts into, which is stirred all the time. The specific heats are $2100\ \text{J/(kg K)}$ for ice and $4200\ \text{J/(kg K)}$ for water, and the latent heat of fusion of ice is $3.36 \times 10^5\ \text{J/kg}$. How long after switching on is all of it water at $20\ ^\circ\text{C}$? Give the answer in $\text{s}$.