Windows 8 will enter the Release Candidate stage in late May or early June, according to a post made by winunleaked.tk over the weekend. This would mesh with an earlier rumor from Bloomberg that the software will be finished in the summer, with retail availability in October.
All these dates are unsubstantiated rumor at this point, but they're not at all surprising. Microsoft has been clear that the development of Windows 8 is following the same track as Windows 7—initial preview about a year before release, public beta in the first quarter, release candidate mid-year, and a release in fall.
The timelines aren't, however, identical. Windows 7's preview came in October 2008, with a beta shipped at CES in early January 2009, the release candidate in early May 2009, and the final build completed in late July 2009, with retail availability in October of the same year. Windows 8's preview was released in September 2011, but its beta didn't ship until late February of this year, making it a little slower to reach the same developmental stage as Windows 7. A June release candidate would similarly be a little behind the timetable laid down by Windows 7.
While Windows 7 and Windows Vista both had three months between RTM and retail availability, there's likely scope to compress this slack time. Windows XP, for example, had only two months between its August RTM and October retail release. Windows 2000 similarly had two months between a December RTM and a February release. If Windows 8 RTMed in August, per Bloomberg's summer estimate, an October launch would be a bit tight, but well within reach.
