Once the thermostat switches over to battery, it will have enough battery power to last a couple of hours. Then, it will completely shut down. It does try to make any necessary changes to its programming in an attempt to keep it running as long as possible, but once the battery dies it can no longer control your heating and cooling.
If you live in an area where power outages are frequent, or you are generally concerned about your Nest thermostat losing power, you should not rely on its battery life. Since it is only designed to operate for a short time on the built-in battery, usage on the battery is not meant for heavy or long-term use. If this is a concern for you but you still want to have a smart thermostat in your home you should consider investing in an Uninterruptable Power Supply (UPS) or generator that will ensure your peace of mind.
