No time limit, 6-month warranty included.
Cannot be used after a system format.

Instructions:

You must use the license key within 12 hours.

On the activation page, find and click "Change product key" or "Enter license key".
Enter your license key in the box.
At this stage, you should receive the error code 0xc004c008.
Close the error screen and open "Run" from the Start menu.
Type SLUI 4 in the Run window and press Enter.
Then select "Activate by phone".
In the window that appears, select any region and click Next.
A long installation ID will appear on screen.
Go to aka.ms/aoh in your browser, log in with your account, and open the Windows activation option.
Enter the long numbers (installation ID) shown on your computer into the fields on the website and follow the instructions.
The system will give you a confirmation code. Enter this code sequentially into the boxes on your computer.
After entering the codes, complete the activation. If it says it cannot be activated by phone, restart your device and it will become active.