Microsoft just revealed that Windows 10 will be released this Summer. An exact date was not yet given. The news was revealed by Windows Chief, Terry Myerson.
“We continue to make great development progress and shared today that Windows 10 will be available this summer in 190 countries and 111 languages."

Don't forget that Windows 10 is a free upgrade for Windows 7, 8, and 8.1 users. This free offer will be available for one year after Windows 10 launches. So if you want to keep Windows 10 forever for free, you'll need to make the upgrade within that year window.

You can find out some additional Windows 10 release tidbits from the Windows blog.