Is Windows 8 Pro usable without activation?

Solution 1:

With all the uncertainty and doubts, I decided to conduct an experiment. I installed a clean Windows 8 Pro copy (a copy that I own and with a key that wasn't used anywhere yet) on a computer without Internet access. Here's what I found.

You don't have to activate Windows 8

It is true that the installer requires you to enter a valid Windows 8 key before you can continue with installation.
However, the key isn't activated at install time and the installation goes just fine without Internet connection (or calling Microsoft).
After the installation finishes, you can create your local account (connecting Microsoft Accounts is unavailable because there's no Internet connection) and you land in the Start Screen, which isn't any different from the Start Screen of an activated copy.

Don't expect your life to be easy, though. Microsoft will try to encourage you to activate your copy of Windows 8.

Reminders and Limitations

The first reminder you'll get will probably be an Action Center tooltip in the Desktop Mode:

You can safely ignore this tooltip and nothing bad will happen.
Another reminder you'll probably see quite often is every time you open the PC Settings screen. When you do, you're greeted with a new section:

Again, you can safely ignore this and click any other settings section on the left-hand side of the screen. Except that...
Personalisation settings section is disabled:

The disabled settings include:
- Your lock screen wallpaper and apps
- Start Screen colour and background
- Your account picture
The message says:

You need to activate Windows before you can personalise your PC.

I heard there are some workarounds for this, but as this was not a part of my experiment, I will not describe them here.
Every couple of hours, you'll see a full-screen activation request:

To dismiss it, you have to click Go to PC settings and then you're back to the screenshot from point 2. Still, you can exit this screen normally, with Win or Win+D, or any other way you like.

I wasn't able to pinpoint the exact frequency with which this overlay appears, but it's certainly more than 5 hours and less than 7 hours, so I guess it appears every 6 hours.
When you try to enter a command to show the status of activation:
```
%windir%\system32\cmd /c slmgr/xpr
```
You get a not very informing message, saying that Windows is in Notification mode:

EDIT: As vhanla notes in the comments, if you run:
```
%windir%\system32\cmd /c slmgr/dli
```
you'll see:

Name: Windows(R), Professional edition
Description: Windows(R) Operating System, RETAIL channel
Partial Product Key: XXXXX
License Status: Notification
Notification Reason: 0xC004F009 (grace time expired)

But even though the grace time is already expired from the start, Windows continues to work correctly.

So... I guess that's it. There are no other differences from the activated copy of Windows 8 Pro that I found. Of course, because I didn't enable Internet access, online-related activities like installing new apps from Windows Store or obtaining system updates obviously don't work.

TL;DR

You can install Windows 8 Pro without an Internet connection
Windows 8 Pro without activation is usable
There is probably no time limit for activation of Windows 8
You'll get occasional reminders to activate, but they are safe to ignore
The only thing that's disabled is Personalisation Settings screen.

Solution 2:

System Restore is Disabled (greyed out) while un-activated. (Windows 8 PRO Full RTM)

I also noticed Auto Play settings are all set to "choose a default", not sure if this default settings for an activated system. They can be changed however.

enter image description here

Solution 3:

Due to a re-installation, my Windows 8 doesn't activate. It has been working without activation for a year now. The prompt comes up every couple of hours, but it seems to be actively stopped by streamlining programs such as Razer. You're fine to go indefinitely, methinks.