I have gone a few days and come back on threads and go to last unread would be 150+ posts away from the end. It has worked without issue. In fact if you go to last unread and it is not on the last page and you leave the thread and go to last unread it will take you to the next unread page. Essentially it figures the last post on the last page you saw is your leaving point. So, if you had 5 pages to read you could go to last unread 5x and go to each page.