What happens to human mind during solitary confinement?

I just recently caught up on the solitary confinement affair.
I've read that there's been study where graduates were paid to sacrifice themselves.
And none of them lasted a week, and reported having problem in thought process and having problem about thinking anything but the thing that soothed them (dogs and cubes(?)). And that was just a week.

Afterwards I've seen some playthrough of a video game. It was "mental asylum", but they abused their power, they brainwashed people into bad stuff. Typical psychological horror game.

So I was wondering, how long does it take in solitary confinement (regular prison, without brainwashing) to have what effect?

What would happen to Average Joe, in 1st week, 2nd week, 3rd week etc. ? I'm not a weird psycho. But I am wondering, how bad can be in be, and in which amount of time? What is typical thinking after what amount of time?