How long does it take for Salt water to freeze below 0?

Since the salt is already a solid, it cant really "freeze" the water in it freezes at water's freezing point normally, but depending on the salt it lowers the frezing temperature of the water.

For example:

*Sodium Chloride lowers freezing point of water to 15 degrees Fahrenheit

*Potassium Chloride lowers freezing point of water to 20 degrees Fahrenheit

So, depending on the salt type, the water around the salt will freeze at different times, it also depends on the water's thermal energy and the temperature of the area around the water.