Answer
About 10000 s, or 3 hours.
Work Step by Step
A football field is roughly 100 m long and half as wide, so the area is about 100 meters by 50 meters, or $5000 m^{2}$.
Assume the mower has a width of 0.5 meters and that the person walks at about 1 meter per second. Each second, the person can mow $0.5 m^{2}$ of the field. The total time needed is $\frac{5000 m^{2}}{0.5 m^{2}} = 10000 s$.
