What does "sent to the hole" mean in prison, how long for?

The "hole", also known as the SHU (special housing unit) is prison inside prison. The inmate is in a one or two man cell with zero access to the outside world except for two one-hour exercise periods where you are in a cage outside. It is strict, high security with 24/7 monitoring. Inamtes get three square meals per day and three showers per week, one 15-minute phone call per week and a very abbreviated commissary list. It is not the place that any inmate wants to find themselves.