Normal paperbacks normally begin to appear around the middle of the month before there announced publication month (October books tend to be out in middle September etc.).
^^^ It does all depend on where you live of course. I noticed in the Destiny speculation thread that the British orders are now also arriving. Over here, the (Dutch) stores usually trail the US releases by two weeks. I'm not sure how it is for other "overseas" areas. (And on a side note, I don't mind the waiting. It's the temptation to read spoiler threads that's the issue for me...)
Yeah, I was talking about the U.S. Amazon.de, where I get my english language books, most of the time has a delay of at least three weeks.