If they're good quality, they should last up to 2 years with normal use. In the event you repeatedly get low battery messages, or if there’s another power-related message, you may need to install a C wire to provide consistent power to your thermostat and prevent the backup batteries from draining. Some systems, including heating-only, cooling-only, zone-controlled, and heat pump systems, require a C wire or the Nest Power Connector. Go to g.co/nest/cwire for details.
