CMOS battery-how long its life?

[Nasikh]

I have been using my X220 since 2011 (9 years), at least 95% of the time in an Ultrabase 3 (i.e., on AC) with the battery in the machine. To this day, I have never seen any sign that the CMOS battery is not working. How long can I expect it to last, given that almost all the time I am working on AC? Should I put in a new one, preemptively, before the original one dies?

[RealBlackStuff]

The usual CMOS life expectancy is about 5 years, but can be a lot longer.
As long as the voltage is between ~2.95V and ~3.3V, you're good to go.
If/when you replace it, check that there is no password set in BIOS (or that you know it)!

[MikalE]

As far as I know the battery in my T520 has never been replaced either and it's still ticking along. That goes for my two T510's and T500 too.

[w0qj]

The CMOS battery in our T410 is ticking alive and well

From our empirical observations, it is when you fully discharge your main battery, and then let your computer sit there unused (and unplugged) for months at a time.
ie: computer uses the CMOS battery to maintain itself.
That's when your CMOS battery is in danger. Just our two cents.

[kfzhu1229]

The CMOS battery of my T42 and T43/p's all died about 1-2 years ago. So about 12-13 years in my usage.