Hi,

There two things to consider:

1 – The battery has a live cicle. In the body of the battery has a date.

2 – The battery has an Ah capacity. You can estimate the best time dividing the Ah capacity per CPU current usage (see catalog ST70 -> S7-x00 chapter -> CPU characteristics -> current consumption -> back-up battery -> back up current typical).

For example: CPU 414H has 550 micro Ampere consumption and a battery has 1.9 Ah, so 1.9 h / 550 * 1000000 = 3454h