This depends on your website type:
Paid website (Premium)
The notice period is 2 weeks up to the end of your contract term.
If you have not made the termination within the 2 weeks notice period, your paid website will extend for one more period.
Free website (Basic)
If you have a free website, you can delete (terminate) it anytime by yourself.
To delete your free website, please go to "Settings" => "Website" => "Delete website" on the menu.
Make sure you are in the interface of the website that you want to delete. This is very important, especially if your account contains multiple websites.
Please note that your entire website and all contained files and data will be deleted permanently and irrevocably.
If you want to delete your complete account (and not just your website), please follow the steps as described in this help article: