Clams can only live out of water for 2 to 3 days.
To keep clams alive and fresh before cooking the clams you should place the live clams in the refrigerator in an open container, covered with a moist cloth or paper towels.
This will keep the live clams fresh and alive for up to 3 days before they die off and should not be eaten.
Never place the clams in an airtight or sealed container as doing so will kill them and you do not want to eat clams that are dead until you kill them first before cooking.
The clams should only be killed at the time you're cooking them because dead clams left out grow bacteria and can make you sick.