For the duration of his sentence he would be in a windowless cell by himself for 23½ hours per day. He would be let out of the cell, but not allowed out in the prison yard, for exercise and medical checkups, etc. His food would be brought to him. He would not be allowed to even see another prisoner.
What you describe is almost exactly the conditions that innates have at the “supermax” prisons. Two well known people that were sent to a supermax are Timothy McVeigh (before he was executed) and Theodore Kaczynski (the Unabomber). There is a supermax in California so no, I don’t think they consider it cruel and unusual punishment.
A federal judge in Wisconsin has ruled that the conditions at our Supermax, Boscobel, do violate the 8th Amendment. It’s been a while so I don’t recall all the specifics; Googling “Crabb Boscobel” brings up a lot of hits which someone with a higher level of interest than I can sort through (Barbara Crabb is the judge’s name).
And yes, I would consider locking someone in a room forever to be cruel and unusual.