The Smart Sock has about a 18 hour effective battery life. Depending on use, distance to Base Station, and other factors, this battery life can change.
Whenever the Smart Sock is not in use, simply plug it back into the Base Station until you use it again.
This will ensure you don't misplace the Smart Sock, and that it will be fully charged and ready to go every time you use it.
If the Smart Sock is not on your baby, the Base Station will be likely to give you a yellow notification since the sock is not able to get a reading.