Traditionally there's a major Fedora Linux release every six months -- give or take some weeks depending upon the ever-frequent release delays. With Fedora 20 having made it out in December, one would anticipate Fedora 21 would ship in May or June. However, it doesn't look like it will end up being released before August.
Jaroslav Reznik of Red Hat wrote a new blog post today pointing out this information, which was pointed out some time ago by this FESCo ticket. The additional time is for teams to focus on their outstanding work and to work on tooling for better release automation.
Another change with Fedora 21 is that there will no longer be official release names / code-names... Recent Fedora releases have gotten a bit out of hand with names like the Beefy Miracle. The Fedora Board is no longer getting involved with release names but will leave it up to the community and working groups if they want to come up with their own name.