According to the official introduction, under normal circumstances, Apple Watch Series 3 can be used for 18 hours after being fully charged. Specific usage scenarios include:

18 hours, viewing time 90 times, receiving notification 90 times, using app for 45 minutes, exercising, and playing music from Apple Watch via Bluetooth for 30 minutes. The usage of Apple Watch Series 3(GPS) includes: connecting iPhone via Bluetooth during the whole test time of 18 hours. The usage of Apple Watch Series 3(GPS+ cellular network) includes: within 18 hours, connect to LTE network for 4 hours, and connect to iPhone * * * 14 hours via Bluetooth.