I remember reading it on MSDN a while back. Not sure where. Someone expected two release candidates before final release. I think it depends on how bug-free RC1 turns out.
From April:
http://blogs.msdn.com/ie/archive/2006/04/24/582546.aspx
What’s next? Windows Vista Beta 2, and then Beta 3 of IE7, release candidates, and then a final release before end of year.
From August:
http://blogs.msdn.com/ie/archive/2006/08/24/715752.aspx
Depending on your feedback, we may post another release candidate. We’re still on track to ship the final IE7 release in the 4th calendar quarter.