How long do you allow your not yet requested books to stay on your shelf?  Do you have a limit?

My shelf is pretty small (I limit it to <25 books), so basically forever. I mostly trade WL books and don't add anything to my shelf that has a ton of copies.

I used to leave them on there without any attention but I was getting overrun by books and that's when I started using the book bazaar. Similar to Caviglia, books with tons of copies go right on my unpostable book bazaar list. 

I think my shelf is around 35 books. 

I only post WL books at this point. I don't like having any just hanging around. If it's not WL then it goes right into the donation box.

When I had more space I would keep them for a few months.  But even then I rarely posted anything with more than 50 copies in the system.  If you post wish list books regularly then you don't need to keep a large bookshelf to keep yourself in credits.  Book clutter stresses me out so I like to get them out of my house as soon as I've read them or decided I don't want them anymore for whatever reason.

There's no right or wrong answers to this stuff, you just do what works best for you. Some people don't mind having hundreds of books on their bookshelf, some do.

I have kept books on my bookshelf from the time I posted them, some as long ago as 2007, until they were requested. However, with a large reserve of credits now, I am on the verge of purging/donating all books that have more than a few copies listed. Tired of storing them and with the price of posting continually rising, it no longer makes sense for me to maintain a large bookshelf. Will be glad to have a smaller bookshelf with just more obscure books.

I currently have 150 books on my shelf, space to store 20-30 more so as long as I have physical space in my guest room closet, they stay.  Last month I had two books requested that had been on my shelf for over a year, but I'll let them sit as long as it takes.

   Even if a book has hundreds posted, a friend here could order your copy directly or someone ordering a book via FIFO off your shelf could 'add more' and take another that's been sitting there a  long time.

When we find a nice shelf or DVD shelf we buy it and make room.  I think we are out of room for more shelves but I have space for more DVDs. I forced myself to halt more book buying unless it's a really special book.  I have a lot a books waiting to be read so that's why I stopped buying books. 

I should probably add that we have hundreds (if not thousands) of books in our house, but I assumed you just meant the ones I intend to get rid of.

ETA - to be honest, I never seem to have any books that have hundreds of copies in the system, so it rarely comes up as an issue.