Good question. The condition of the electrolytic capacitors are also a matter to consider. It's not good for them to be left on the shelf for a long time. If we are talking about peak condition, I think an hour or two a year might be a little too rare.

On the other hand, what's the point of keeping them, if you only run them for an hour or two a year, just to make sure they still work? You should have fun with them while they last anyway. That's why I started slowly downsizing my collection, only trying to keep stuff I'm attached to or that are spare parts for the builds I'm using regularly. I started thinking that it's a waste to sit on stuff I'll never use past the test runs I did after acquiring them. Having all those stuff that could make others much happier than me and go to e-waste if I kick the bucket are already as good as e-waste if left unused.

Good quality electrolytics can reform if powered up in a certain way and then used enough. I would think it's not so much about how often, but the process used on which kind of caps. I would say there might not be a simple solution for more complex situations. If there is a good study behind this for PCs, let me know! I've only seen a few cases.

There are some argument to be made that the power cycling put wear on components too. Again not an exact science. So if you're powering on more to try to save your caps, you might be shortening some other thing's life. So I don't know what harm really to let things sit for a long time. But it's hard to see if powering once a year will help. So either your using it regularly, or your most letting it sit. Not sure if any balance can be found!

In terms of many batteries, age and lack of use it pretty much kill them no matter what. Not sure that powering on will matter. You'll pretty much have to replace it eventually. The oldest RTC battery I've seen latest just short of 30 years. It was a high quality panasonic lithium, which are still made. So I bought another 😀 This type doesn't charge of course, so it won't matter turning it on.

I worry about my 1999 iMac's battery so it gets powered on once a month at least. I had another of these machines, which sat in a cupboard unused for several years, and died because the battery leaked. The iMac is also constantly connected to the mains. (It only gets unplugged when we go away for a few weeks.) I don't know if this makes a difference.

My other machines don't have these scary barrel batteries so they are less concerning.