When you 'delete' a site it is 'soft' deleted so that you can restore it if you realise you made a mistake. This means the name is still reserved for 30 days or until you 'hard' delete the site by going to https://sites.google.com/site/sites/system/app/pages/meta/dashboard/deleted and deleting the site from there.
If a Site Owner navigates to the URL for a deleted site, they will be prompted to "Restore Site" or "Delete Permanently" the site. If the "Restore Site" option is selected, the site will be restored to the same state it was in at the time it was deleted.
Selecting the "Delete Permanently" option will permanently delete the site, leaving no option to recover in the future. This is a permanent option that cannot be reversed.
For more information, see the official Google announcement concerning this feature.
For a list of your deleted Sites, go here:
or, for a Google Apps Domain
https://sites.google.com/a/.. DOMAIN NAME ../sites/system/app/pages/meta/dashboard/deleted