How long does ice take to melt in water at 80 degrees?

Answer 1

This is entirely dependent on the mass of ice, its thickness, and surface area of the ice that interfaces with the warmer water (it will melt much faster, of course, at 80 °C than at 80 F°).

There is also a question of the pressure at which the mixing is done, and, more esoterically, of the specific type of ice, as there are different forms of ice other than what we normally use.

Experimentally

A one-inch cube of ice in a 10 gallon tub full of water at 80 °F melted to invisible size in about 6 minutes. This is because the cube was unable to appreciably cool the larger volume of water. (This time can vary if the cube is stationary, or is moving in the water, which keeps the interface temperature more-or-less constant.)