You can also read more about the Release Candidate on the Joomla dev blog
here.
As mentioned in the announcement of the 1.5 Release Candidate we expect a second Release Candidate and we will try to get it done within 2-3 weeks. I just want to point out that we are trying, saying 2-3 weeks is no promise (remember holidays are coming ). If we need a third Release Candidate is still not determined. From now on Quality and Testing is in the lead and will be the driving force behind the RC2. The developers of course will push forward to RC2, but we also will start thinking on the actual start of the development of Joomla! 1.6. This will take some time also because of the upcoming holidays, but it is something we are starting to think about