Since I live in the central time zone I start seeing new content in the music store late on Monday night. And by early Tuesday morning all the music is available.
TV shows are another matter all together. Some of them don't show up for several day's after the original show aired. It seems to be random.
Movies regularly begin showing up early in the evening on Monday. For example The Dark Knight was available to download at just after 10 p.m. CST.