A plant as simple as grass will have a CN ratio around 20:1 when freshly cut. Within a few day, depending on the weather and environment, it can dry out and become a brown (hay) with a CN of 40:1. There are exceptions. Some plants, cactus for example, can stay alive and hold on to nutrients for extended periods even when chopped to bits.
Whatever you are working with, work with it quickly. I read some time ago that manure, applied to a field, will lose something like 40% of it's original N if not tilled into the soil within a day.
I'd say let your eyes be a guide. You can see the plants going from green to brown. The thing is, if you have too much "green" stuff, there's a danger your compost will get gloppy and smelly. If you have too much "brown" stuff, there's a danger your compost pile will just sit there and not do much. Nitrogen is sort of the spark, but carbon is the fuel for the fire. Of course, if your compost pile is just sitting there, the most common reason (in my experience) is that it's too dry. Wet as a wrung out rag is how I like to keep a compost pile.
I think the nitrogen can just run off into the soil. I think this is why chop and drop works. Permaculture rarely calls for any sort of tillage. If your soil is healthy, you got these great tillers called earthworms, who will grab plant matter and pull it into the soil.