Your Withings Steel HR can operate for up to 25 days on a single charge, depending on your use of the continuous heart rate mode. Using the continuous heart rate mode all the time instead of just during activities will deplete the battery much faster.
Note that enabling the Notifications option will also deplete the battery of your watch much faster.
If you don’t charge your Withings Steel HR right away, the power reserve mode will allow you to keep using it for 20 additional days. However, you will only be able to read the time and track your activity in this mode. You will need to charge your Withings Steel HR to use the heart rate mode.
You can easily charge your Withings Steel HR using the magnetic charging cable provided with the watch.