What happens if you are in a cave and avoid having contact with any kind of light in a long period of time?
25 hours. Previous research has shown that persons living in caves shift to a 25 hour daily circadian rhythm instead of a 24 hour cycle. However, they were allowed contact with artificial light when they wanted it. I don't know the answer to your particular question, as such "research" would be considered torture. Perhaps some data can be gleaned from research involving those with blindness.