How long are invitations kept?

When I invite someone to an organization, they get an email saying:

“If you do not wish to join this organization, you can safely ignore this email.”

In the Admin-Backend I can see all invited users. But how long are these invitations valid? Is it possible to automatically clean up all ignored invitations after e.g. 6 months?

Okay, it seems they are kept indefinitely until they are accepted or manually deleted.

As a work around I was thinking about deleting the entries in the database directly but that seems to be a more complex issue.