> No, sorry, I was referring to your whole opening section, which was a response to my question, "How can anyone possibly make a reasonable, intelligent, a priori assessment of how long that data will be useful for that kind of purpose?"f

Doesn't matter.

It's not your data.

It's their personal data.

You may use it as long as it benefits them; As long as they would want you to.

Why do I want a site to have my email address? To send me notifications I've requested? To facilitate a password recovery process I initiate? To send me marketing material that I am interested in? Surely it's obvious that if I change my mind, that's up to me.

Why do I want a site to have my IP address? To help protect my account? Sure. For blacklisting addresses that try to log into my account with an invalid password? Of course.

See? Specific examples are easy. Enumerating them is hard though, which is why European courts don't do that. They rely on organisations like the ICO to field questions, make a judgement call, and publish guidance for frequently asked questions. But for the most part, it's just common sense.

> My point is that even something as "simple" as deciding how long you should retain some data that you originally acquired and have used for some obviously legitimate purpose is not necessarily an easy question.

You should keep it for as short a period as possible.

The longer you keep personal data, the longer it is at risk. Remember you're also responsible for keeping that data safe. If you get hacked and lose control of that data, you're responsible!

> You've linked to a few pages on the ICO's site throughout this discussion. I note in passing that reading and understanding them fully would take many hours, even just looking at the high-level guides and checklists, and having done so, they still leave numerous subjects open to interpretation or judgement where someone would probably need real legal advice to find out where they might stand in practice.

Understanding how to pay tax in the US correctly takes much more than "many hours", and very often requires professional advice.

However very few people worry about tax on millions when their income is under $100; Very few companies need to worry about how to handle millions of requests for erasure when they don't even have any personal data.

But some prudence helps: Simply "not keeping data you don't need" is the ICO's advice. It's also best practices for IT security.

Take the few hours to understand it. If you've got specific questions, I might try to answer them, but you're unlikely to have more than a few for your specific business case, and you'll find legal advice for those questions cheaper than tax advice.

> I would argue that expecting even a good faith effort at compliance from many of those smaller businesses is unrealistic

Well, good luck with that.

My experience with European regulators like the ICO is that they're not going to be amused by your argument.

Given that most of this has been law for decades is a big part of why I think it's not unrealistic.

> That then means that any of us who are concerned with trying to do the right thing and do want to understand our real legal obligations will automatically be at a disadvantage.

Then do the right thing: Treat the data as the person would want it treated. Make sure you are proactive and do your best. Don't worry so much that someone else is going to treat people a little worse so you need to abuse people as much as is legally permitted.