A leaked document has verified the development of Windows 9 and Windows Phone 9. The document, published by Myce.com, talks about the builds being released for preview at some point between Q2 and Q3 2015. While the date range is very broad, it could very well mean that Windows 9 could be hitting both PCs and phones in less than a year.
Windows Phone 8.1 will be rolling out with a whole slew of new features, so there's no telling what Windows 9 will bring. Some rumors have suggested it will lead to Microsoft completely doing away with Windows RT, and replacing it with Windows 9 on tablets. We have a while to wait yet, but the Windows Phone 8.1 release should be enough to tide us over in the meantime.