This depends on the GPS interval usage that you set up.
1. Tracking every 30 seconds: Fast positioning mode and standby time is 8 hours.
2. Tracking every 10 minutes: Standard positioning and standby time is 48 hours.
3. Tracking every 1 hour: Power saving mode and standby time is over 72 hours.
Please keep in mind, that the watch should be switched off when its not used to perserve the battery life. Especially at night time, unless you're wanting to track a sleep walker :)
It should also never be left on charge. Do not charge for more than 3 hours at a time.