The time it takes for salt water to freeze depends on the amount of salt in the water, the temperature of the water, and the volume of water. The more salt the water has, the lower its freezing point. Smaller volumes of water will freeze more quickly than larger volumes.
Sea water freezes at a temperature between 28 and 29 degrees Fahrenheit. The freezing point for sea water is lower than it is for fresh water--which freezes at 32 degrees Fahrenheit--because of its greater density. 500 ml of saltwater (containing 1 tsp of salt) will take approximately four hours to freeze in a home freezer.