Once a year if you have a paid developer account, which is $100/year. It’s otherwise every 7 days and capped at 3 simultaneous apps*. And in either case you need a Mac or Windows machine available as well. I suspect it’ll be an improvement for anyone without a paid developer account.
* - At least, according to https://sideloadly.io/ - I haven’t touched sideloading for a few years (with AltStore + AltServer).
It’s an educated inference - I don’t believe that requiring a paid dev account to sideload would satisfy the EU, and I don’t think a system that requires a paid dev account would be meaningfully different from the current state of things.
But then again, I will be surprised if the EU allows Apple to charge a commission on fees related to apps distributed outside their store, and they’re doing that anyway, so who knows.