How often should I replace the batteries in my TH6220D?

You should replace the batteries once a year, or before leaving home for more than a month. The 'Replace Battery' warning will begin flashing on the display approximately two months before the batteries are fully depleted, so you should install fresh AA alkaline batteries as soon as that warning appears. If batteries are replaced within two minutes of removal, the time and day will not need to be reset. (Page 17)