On a sustainable diet, theoretically they would live a semi normal life expectancy. Physiologically, all of their needs would be met, however psychologically, this is a different story. By nature, humans are social creatures who depend on social interaction. If a person was never to have seen the outside world, they would take on an Allegory of the Cave mindset, believing that this is the world.
However, in the description that you have provided, they would have been placed here after an understanding of social structure and daily function. Even if they did not kill themselves, I believe that even the strongest people would develop psychological disorders. An example of this would be like the movie Cast Away, where Tom Hanks has conversations with the Wilson Volleyball. They may hallucinate, have delusions, and could potentially even develop Dissociative Identity Disorder.
Another theory would be that this human may potentially lose the will to live. They may not even have ideas of killing themselves, but would eventually wither and cease to live through a lack of humanity. The brain is a powerful thing, and as mentioned before, social interaction is imperative to human survival.
What you describe is actually in use today, although not for use of punishment. Sensory deprivation tanks are used for relaxation, although studies have found that even short periods of time can produce hallucinations in individuals, equally across those who are prone to hallucinations and those who are not. I hope this answered your questions about the effects your tank would have on an individual.