Battery charge time depends on several factors, including ambient temperature, and the state and condition of the battery.
Nest Doorbell (battery)
Typical: It takes approximately five hours to fully charge the battery using a compatible 7.5-W AC adaptor (sold separately).
Important: To help prolong the life of your doorbell's battery, it will only charge to 75% if the doorbell is wired to the doorbell system. Before you install your doorbell, you can use the USB cable to charge the battery above 75%. However, after the doorbell is wired to the system, the battery will drop down to 75% and stay at that level or below.