From Fedora Project Wiki
Time Based Release Schedule
Fedora follows the precedence of Red Hat Linux in having a new release every six months. HavocPennington offers a historical perspective on this decision. GNOME started following a time based release based on the ideas and success of Red Hat Linux and others distribution following Fedora having adopted a similar release cycle too. Several other major components including Linux kernel, Openoffice.org, Xorg have started following a time based release schedule too. While the exact release schedules vary between these components, the upstream projects, the interactions between these components and Fedora makes a six month time based release schedule, a good balance.