When an item is deleted from a SharePoint Online site, it is sent to that site's Recycle Bin (or first-stage Recycle Bin). If an item is deleted from the Recycle Bin, it is sent to the Site Collection Recycle Bin (or second-stage Recycle Bin), where the SharePoint site collection administrator can view and restore it. If an item is deleted from the second-stage recycle bin, or if the item exceeds the retention time, it is permanently deleted.
The retention period begins when something is first deleted and does not change if the deleted item moves from the first-stage to second-stage Recycle Bins.
When a user deletes an item it will be retrievable for 93 days before its permanent deletion.
For information on restoring deleted items from the first-stage Recycle Bin, see the following Microsoft support page:
Restore items in the Recycle Bin of a SharePoint site
If a subsite is deleted it will automatically go to the second-stage Recycle Bin and remain there for 93 days, after which time it will be permanently deleted.
For information on restoring deleted sites or items from the second-stage Recycle Bin, see the following Microsoft support page:
Restore deleted items from the site collection recycle bin
SITE COLLECTION DELETION
If an entire site collection is deleted it will remain in the SharePoint Online Administration Center Recycle Bin for 93 days, after which time it will be permanently deleted. The SharePoint Online site's administrator can access this recycle bin in order to view the deleted site collections therein, as well as the days left on each of their retention periods.
For information on restoring deleted site collections, see the following Microsoft support page:
Restore a deleted site collection