If solid stones a simple rinse will do. If porous like in coral based stones, it will take longer and the water should be replaced a couple times during soaking. A simple test is to taste the water and if salty, replace and soak some more.

Be aware that coral will buffer water it comes in contact with and turn it alkaline. Also, all stones should be bigger than the frogs mouth to reduce possibility of frog eating a stone while hunting for food. Good luck !