The world was created in 7 days, or so the Bible says.
is almost equivalent to
The world was created in 7 days. At least the bible says so.
You're implying that the Bible is the only source (or one of few sources) that says so, and that you're not convinced.
The world was created in 7 days, as The Bible says.
This means that we believe in what the Bible says. And it says that the world was created in 7 day. And so it was then.
The world was created in 7 days, or so-called The Bible says.
This sentence is incomplete. One way to complete it would be
The world was created in 7 days, or so the so-called Bible says.
This is the same as your first sentence, except that you've added the epithet "so-called" to the Bible. In one sense, "so-called X" is used when one thinks that the name X, although widely accepted, is for some reason inadequate for the entity it is supposed to denote. Whether or not "the so-called Bible" makes any sense is up to debate.