The kernel has a release cycle of every one or two weeks (rc-release) depending on the changes and quality of new code that Linus Torvalds gets. Interesting enough, Firefox is adapting to that method of release scheduling as well. Obviously there is no such thing as a time-scheduled release. There are just incremental upgrades and new features. New features obviously always have to go through several incremental stages.
This is an excellent paper about Linux:
from the paper:
Distributors like Red Hat, Novell, and MontaVista have a clear interest in making Linux as capable as it can be. Though these firms compete strongly with each other for customers, they all work together to make the Linux kernel better.
Companies which are not in the information technology business can still find working with Linux
beneficial. The 2.6.25 kernel included an implementation of the PF_CAN network protocol which was contributed by Volkswagen. 2.6.30 had a patch from Quantum Controls BV, which makes navigational
devices for yachts. These companies find Linux to be a solid platform upon which to build their products;
they contribute to the kernel to help ensure that Linux continues to meet their needs into the future. No other operating system gives this power to influence future development to its users.