There is no such thing as being 'sentenced to two years' solitary confinement'. Prisoners may be kept in the segregation unit for a maximum of two weeks. After that, they have to be reinstalled on a wing.

Solitary confinement isn't part of a sentence, but used to segregate a troublesome inmate for a short time to defuse certain situations.
There are strict rules about segregation. Daily visits by the Governor and the Chaplain and air and exercise for an hour each day.

I know nothing however about prisons for the criminally insane (eg Broadmoor) where isolation may or may not be advised.