According to the CDC (Centers for Disease Control and Prevention), the average length of stay in a hospital for mental illness was 7.2 days. That was in 2010 in the US. (http://www.cdc.gov/nchs/fastats/mental-health.htm). I have no reason to expect that hospital stays are getting longer. In fact, they’re likely getting shorter. A 2006 study (http://www.hcup-us.ahrq.gov/reports/statbriefs/sb62.jsp) found that the average mental health stay in a hospital was 8.2 days.
The reduction in hospital stays took off during the Reagan administration. The idea was to shift care of the mentally ill to the community instead of long-term hospitals. Unfortunately things didn’t work out quite as expected, but nevertheless the trend continues. Managed care has a lot to do with the trend now since costs are cut. Check out this article if you’re interested in what has happened with mental health hospitalizations: http://content.healthaffairs.org/content/26/6/1548.full

(A mental health hospital in Michigan: http://www.mhweb.org/wayne/reuther.htm. Not quite the idyllic image so often seen in media.)
So, what does that mean to you as a writer?
First off, it means that if you have a patient hospitalized for months or years, you’ve gotten it wrong. I love the show “Criminal Minds” but Reid’s mother being hospitalized for years and years just rings false. Much more realistic would be to have her bouncing in and out of a psych ward, living off disability and struggling to make ends meet. That could easily be done by having Reid help her out financially. This would give Reid a realistic source of stress.
BTW: there is a way to have your character be hospitalized long-term. Have him found to be “insane” and set by a court to a locked mental health hospital. The truth is, the only way to get hospitalized for years and years is to commit a crime and plead insanity and be found insane. That’s how John Hinkley, Jr, the man who attempted to assassinate President Reagan, has wound up in St. Elizabeths Hospital in Washington, D.C.
Next time we’ll look closer at voluntary and involuntary admissions to a psych unit or hospital.