New ports generally vary in the time it takes to get one going from start to finish. It depends on several factors (but not limited to):
1) People who own a device that doesn't run Rockbox, but are interested in getting a port going
2) Available information about the hardware in a DAP, including CPU, LCD, storage medium, and more
3) Time, dedication, lots of hard work.
That said, all that has been done for the iPod Classic is just some very basic investigation. No actual work has been done. You're best off going on eBay to look for a Rockbox compatible device.