How often should I replace the batteries in my TH6110D thermostat?

Batteries should be replaced once a year, or before leaving home for more than a month. You should also install fresh batteries immediately when the REPLACE BATTERY warning begins flashing, which appears about two months before the batteries are fully depleted. If batteries are inserted within two minutes of removal, the time and day will not need to be reset, as all other settings are permanently stored in memory. (Page 17)