Android 6.0 Marshmallow was released about 10 days ago, and began rolling out to the Nexus devices on October 5th.
But it appears that the most popular custom version of the mobile operating system, is already being worked upon, using the latest source code.
Yes, CyanogenMod 13.0 based on Android 6.0 Marshmallow is already being prepared. Now that the work on the next gen ROM has begun, , it may take a few more months for the first ROMs are published, for the Nexus devices which already have Marshmallow from Google. Still we may be able to see the first nightly version of CM 13.0 in the next few months for the Nexus devices, and by early 2016 for other devices.