How long does it take for Google to delete a website index?

My website has been deleted, but Google still has it indexed.
How long does it take for Google to delete an inaccessible index?

It can take many month before it is totally gone, at least in my experience on domains I´ve tried to de-index.

This is tough to measure but it should not take more than1-2 months.

Google is actually a funny thing because sometimes it takes months to index it and sometimes we need to wait months to the De index it.

Anywhere from a couple of weeks to several months, up to 6+- months in some cases/pages,
Depends on how many pages the website has, how often its pages are being crawled/recrawled, etc...

You can request the pages to be removed via GSC as well, that should speed things up,
Under the removals tab, you can request removing the URLs, either temporary or as outdated content