Is there a limit to how long deleted pages and/or attachments remain in the recycle bin (‘Deleted Pages’ and ‘Deleted Attachment’ tabs in ‘Page Index’ application)?
If no, is it possible to set a retention time for deleted pages and/or attachments?
No limit. You decide when you want to purge it (safer).
Not possible right now. Note that in XWiki 10.11 we’re introducing a button to purge the recycle bin.
No.
This feature does not exist right now. But indeed controlling the maximum size/age of the trash bin could be nice.