How long can I use Windows 10 without activation?

Is there an official "evaluation period" for Windows 10, and how long is it?

There isn't really a proper evaluation period by default on a standard copy, especially since it is free if you have a regular copy and the vast majority of machines compatible are eligible. So, you either have a proper legal copy that is not activated, or, you can download a 90 day trial of Windows Enterprise direct from Microsoft.

How long can one use (from a technical perspective only) a not activated copy of Windows 10?

There is no limitation.

Which limitations are there for a not activated copy and when do they step into effect? I was, for example, not able to confirm that personalization is not possible as seems to have been the case with a not-activated Windows 8.

You get a watermark in the bottom right hand corner of the screen and you can't utilise Microsoft account services such as syncing settings between devices. Other than that, it pretty much works as expected.

My Experience:

Since I changed my motherboard a while ago, I have had an "unlicensed" copy which I haven't been able to reactive. I have to say, other than the watermark, I wouldn't really know that this was the case.

According to the retail license agreement, section 5:

"You are authorized to use this software only if you are properly licensed and the software has been properly activated with a genuine product key or by other authorized method."

So if the software has never been properly activated, you are not authorized to use it, and by my reading, you can't use it under this license. I'm going to assume that they mean "has been properly activated" to apply to a particular installation, and not Windows 10 as a software product, because surely someone has activated Windows 10 ever. It might apply per copy instead, though, so if you activate on one device and then move your license to a new device, you may not need to activate the new device.

So, in my non-lawyerly legal opinion, the official evaluation period is length 0, and you are committing copyright infringement the moment you boot an un-activated Windows 10.

Interestingly, I couldn't find an exception that allows you to use Windows 10 to run your computer long enough to enter your product key and activate it. Any use of an unactivated copy, including installing and activating it, is unauthorized. So pretty much everyone seems to have violated the license terms.