I am toying with an idea which requires a human to be stuck in a dark box for the rest of their life.
They receive regular exercise of the arms and legs, air, food and water and their bodily functions are taken care of by the box.
The only contact they have to the outside world is through a pair of binoculars and hearing trumpets. They have contact with humans, but cannot respond.
How long can someone survive with only movement, sight and hearing of the outside world, air, food and water?
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.
I would say until they sucessfully commit suicide, through refusal to eat or drink, due to the psycohlogical torture. It sounds rather horrific.
But given they do not go insane and all of their physical needs are taken care or, water, air, food, waste removal etc... then potentially as long as most other humans, potentially longer than average as they may be less likely to encounter pathogens, and be run over by a car or other accident.
I think the answers to this question support my point on insanity: How long can a human stand to be underground?
A complete lack of stimulus (such as you're talking about here) fairly readily causes hallucinations. This is the Ganzfeld effect and here's a good ten-minute YouTube video where some dudes try it out and report utterly fantastic results.
This, in like half an hour. I don't know what would happen to someone subject to these conditions indefinitely; I'm not a mental health professional. But if hallucinations set in in less than an hour, I am pretty sure the victim of this torture (because it is torture) would be a complete mess after (say) a year.
Until they die of natural causes (such as a strong disease or old age).
In other words, their life expectancy does not change, since they have all they need.
I am unsure as to whether the likelihood of infection increases, due to the cramped, moist environment, or whether it decreases, due to limited contact with the outside world and a semi-sterile environment.
Life expectancy is reduced drastically. If the person doesn't commit suicide (as covered in other answers), they'd develop dementia rather rapidly because of the lack of stimuli. I'd wager they wouldn't survive longer than 2 years, but that's speculation.
Our mammal brains are rather complicated things that we don't understand entirely, but there's consensus among scientists that it needs training like a muscle to maintain it's full functionality. By putting someone into a box like that, you remove most of the stimuli that could keep the brain in working order for any amount of time.
What's left after all that? An empty shell that lost all brain power but the very basics of survival. Since the brain consumes much of our energy, the unnecessary brain cells will be killed and recycled. That's the definition of dementia.
Changes in eating frequently occur. Caregivers of people with late-stage dementia often provide pureed diets, thickened liquids, and assistance in eating, to prolong their lives, to cause them to gain weight, to reduce the risk of choking, and to make feeding the person easier.[36] The person's appetite may decline to the point that the person does not want to eat at all. They may not want to get out of bed, or may need complete assistance doing so. Commonly, the person no longer recognizes familiar people. They may have significant changes in sleeping habits or have trouble sleeping at all. (Wikipedia)
Lack of hygiene may leed to infections. Lack of appetite may leed to malnurishment. Lack of thirst may leed to strokes or kidney failures. In the end, even though the box provides everything the prisoner needs to survive, they will die prematurely because their impaired brains are unable to utilize these facilities.
