Microsoft Announcement: Windows 10 will be released as a free update on 29 July
This will be Microsoft's last major release of Windows before they move to the new “Windows as a service” system (i.e. automatically rolling out updates when ready)
Windows 10 will be offered as a free upgrade for users of Windows 7 and Windows 8.1 within the first year. So if you qualify there is no rush - you have until July 2016.