Dating Torech Ungol is difficult, except for a comment that Shelob had made her Lair in the mountains before the building of Barad-dûr. This was in about the year II 1000, so evidently Shelob had already made her Lair during the earlier part of the Second Age. She was known to have survived to at least the end of the Third Age, after which she is not mentioned again. After that date, Mordor was once again under the control of the Dúnedain, and they may have made some effort to remove her (though their forebears in the earlier Third Age had not made any such effort, so this is far from certain).