How long should I leave the post for books?  6 months?  If no one is interested, I don't want to keep the books on hold, I am trying to get rid of them! 

It's up to you. Unfortunately with heavily posted books the longer you keep them posted, the more likely they are to get requested.

If you are posting books that have lots of copies in the system, it may take a while for you to get to the "top of the list".  I've been here almost a year and starting a couple of weeks ago I began to steadily get orders for the books I posted when I first joined.  Patience will reward you unless you just want to get rid of the books and don't care about getting credits to order books for yourself.  If that's the case, take the books to your local Senior Center.  They will love getting them.

You can also post a deal of some kind in the Book Bazaar, if you like.  It's one of the discussion forums, on the Community menu. 

It's hard to say.  I've posted books with only a few copies in the system and they sat for months.  Then I've posted books with hundreds in the system and they were ordered pretty quickly.  When I was  posting 3-5 WL books a week, I frequently found that people would add a more heavily posted book to the order using the Order More From Member button.  So posting WL books can also help because many members like to help save other members shipping and so they use that Order More From Member button.

I'll leave my books on my shelf until someone requests them or I run out of room.  I figure if they've already been on here a year, they've made it up the FIFO que a ways--and could be close to being ordered.  Like Kate I've been here about a year and am starting to see some of the books I posted when I first joined being requested.