The LiPo battery life of the 18M varies between hardware A and hardware B devices. Any devices shipped after June of 2015 are hardware B devices.
For hardware B, the battery life is about 20 hours of active operation and 60 hours of sleep operation. For hardware A, the battery life is about 10 hours of active operation and over 20 hours of sleep operation.
See Hardware Hammered Out for more information on hardware versions and for battery best practices please see COBRA Lipo Batteries Best Practices and Procedures.