Microsoft is set to release the first public beta release of Windows 8 on the 29th of February at an event in Barcelona, describing the release as a "consumer preview".
The event coincides with Mobile World Congress which also takes place in Barcelona that week, perhaps giving some indication of mobile devices for Microsoft’s new OS.
Microsoft hasn’t announced a date Windows 7 will be released to manufacturing although it’s widely thought to be around the October time frame. There’s still plenty that could push the date back given the ambitious nature of Windows 8, with an entirely new UI and built-in Store, compared to earlier releases.
Assuming Microsoft follows the strategy as previous public betas, the Windows 8 "consumer preview" will probably not expire until the final version is nearing release. More will doubtless come to light as MWC draws near.