No end date, no prizes, just an ongoing reminder, like with PokPok's Hospital Release Reminder http://www.bookcrossing.com/---/178328 and with Badgerjim's "Doctor! Doctor!" challenge http://www.bookcrossing.com/---/251586 , that these are places where people can really use something to read.
(I personally wouldn't release true crime there or other books that may be upsetting to people in shelters, who may be there escaping abusive situations themselves.)
Dual releases with other Challenges are fine. Please don't post planned releases and only post here AFTER you have actually made a Release Note for the book.
You may not get a lot of Journal Entries from these releases, but nonetheless, I'm sure there will be people who will cherish and appreciate the books, as they may have few possessions of their own.
When I dropped some at a food pantry recently, the staff were very grateful and eagerly looked through the books hoping for some for children. (Also hoping for some in other languages, like Arabic. Unfortunately I couldn't help them out there.)