I think there are too many factors influencing the results (such as Winter's severity, the amount of rain and snowfall, the koi's size, health and stored energy reserves going into winter, the pond's size, type, depth and water quality, water changes and provisions for sufficient off-gassing, etc.) to promulgate hard and fast rules about the length of Winter and its effect on koi.
I'm guessing that's why no one's taken a shot at answering your question?
But just for giggles, let's put some numbers up and let the big guns in the hobby blast away.
Assuming (dangerous, I know) that all the above variables have been maximized in the koi's favor, they should come out:
8-12 weeks under the ice -- Good shape
12-16 weeks under the ice -- Fair shape
16 weeks or more -- Poor shape