It is pretty amazing to see where the development of the Linux kernel comes from. A press release from the Linux Foundation gives the details.
How Fast is Linux Developed and Released?
An average of 3,621 lines of code are added to the kernel tree every day, and a new kernel is released approximately every 2.7 months.
The kernel, since 2005, has been growing at a steady state of 10 percent per year.
Who is Writing Linux?
Every Linux kernel is being developed by nearly 1,000 developers working for more than 100 different corporations. This is the foundation for the largest distributed software development project in the world.
Since 2005, the number of active kernel developers has tripled, reflecting the growing importance of Linux in the embedded systems, server, and desktop markets.
Between 70 and 95 percent of those developers are being paid for their work, dispelling the “hobbyist” myth present from the start of open source development.