This depends on your account type:
Paid website (Premium)
The notice period is 3 months to the end of your contract term.
If you have not made the termination within the 3-month notice period, your paid website will extend for one more period.
Free website (Basic)
If you have a free website, you can delete it anytime by yourself (means "terminate").
In order to do this, please go in the menu to "Settings" => "Website" => "Delete website".
Please make sure you are in the interface of the website, which 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: