Approximately 2-3 years.
We are using high quality lithium battery (such as Sony or Energizer) inside Xronos Clock to keep time if there’s main power interruption. Real Time Clock chip (RTC) uses very little power, so battery should last 1-2 years if your clock never plugged in. Of course chance are it has external power, during which battery is not used at all so it will just discharge at as if it was laying on the shelf (all batteries discharge on their own) which probably will be about 3 years or more.