Old smartphone/laptop, Raspberry Pi, or whatever low power computing platform recording the mains with a sound card connected via an attenuator.
I have yet to own a cell phone.
If I roll something for it, I will just convert the AC to a digital signal and time the pulses. I could just use the relay that I have today that runs from the wallwart drive it.
@bicycleguy
A friend of mine gave me a Fluke 189 that still has the original super cap installed. I've had to replaced a few of them that started to show some oxidation and one that had really high leakage.
After writing this, I thought I've had that meter for a while now and had bought spare caps. Decided to pull it back apart and replace it while I was thinking about it. Noticed the batteries were low and sure enough. See attached.
All four of my old Fluke 18x's now have new caps.
On the flip side, I had a tablet that the PCB was damaged beyond repair after the cap leaked after a a few years. I had two of these tablets and when I looked at the second one, the cap was just starting to go.
So depending on several factors, the life could be a couple years to well over 20.