On average, the battery needs to be charged once every 24 hours but it depends on your pup's activity level and your property.
If you are not getting an average of 24 hours of use, you may be operating on an outdated software version. Upgrade your software here. Or you may be accidentally placing your pup in Tracking mode which can deplete the battery rapidly. To be sure tracking mode is off, in the app, select “Where’s my pet” and tap the magnifying glass to turn tracking off and on.