I can guarantee you it will be no earlier than when ever cm12(or what ever the cyanogenmod team calls it) comes out.
But also, dont forget that the nook color is a 4 year old device, pretty old by tablet and android standards. Im still surprised that they got cm 11 working okay-ish on it(kudos to the development team, amazingly you are still keeping this old tablet alive). And also dont forget that android L completely ditched the dalvik runtime, leaving only the ART runtime for all devices. The last time I checked the development page for the nook, didnt they have issues with ART, something ram related maybe? They are working with a single core 512 Mb s of ram tablet, even cheap tablets nowadays come with a dual core cpu, and at the minimum 512 Mb(most likely 1 Gb at this point) of ram. The nook may be able to run android L, but it could possibly get too laggy for any real use, unless if the development team can get it stable again at higher cpu speeds.