Porting is mostly reverse engineering, or re-engineering things to work with the OS. You can't really say "it will take X months" because there's no real way of knowing without getting stuck into it.

Some devices you can get working rather well very quickly in the community, some things are harder and many people give up before finishing it. These things are complicated, and if you buy a device you expect all the hardware pieces work with the software provided and that takes time.