It's once per year. Source: I have a paid developer account.

But I also need a mac for signing, don't I? Like, a Mac Mini? That's the most annoying part, tbh. I feel fine-ish for paying 8 $ a month for my dev account (if you calculate it that way), but buying additional, expensive hardware? No, not really.

I think that you can rent a mac for a day and do all your signing. Something like https://www.scaleway.com/en/hello-m1/

I’ve had the experience that often weird bugs pop up when signing; often it isn’t that simple unfortunately.

AWS has mac minis by the hour (not cheap though)

See my other reply in this same thread:

https://news.ycombinator.com/item?id=26399788

Long story short, you pay $100/year or $8.33/month for access to a suite of services that make apps frictionless for your users, and easier for you as a developer to offer high end features like authentication, notifications, and sync:

    - App discovery, hosting, distribution, updates
    - CloudKit, iCloud Documents, iCloud K/V Store
    - Push Notifications
    - Sign-in with Apple
    - etc. (NFC is also in the list)

Details: https://help.apple.com/developer-account/#/dev21218dfd6

You do not have to pay anything if you do not want any of those services, however you will have to “refresh” your test app cert weekly or work around that.

I can pay, but, as I said, my apps will expire in a few days, so I would need to rebuild and reinstall them every day to keep them working. The only way to have non-expiring apps is to submit them to AppStore which is obviously not possible, as it's only for me.

If you pay your apps will expire once a year rather than once every couple days.

That's very interesting to know. I guess that's an ultimate solution to my problems then, rebuilding once a year is perfectly acceptable.