There are about 4 near my home. I check them regularly. They are filled with books that stay there for several months. I'll add one when there's room.
If I was running one, I'd have maybe 10x what will fit in it, and constantly rotate them. The ones that don't move within a couple months go to the thrift store.
P.S. the books I add are scifi. They never last more than a week :-)