I landed in the psych unit during one of my jobs. I just told my boss i was in the hospital and that I did not want to discuss the details. (Looking back, it surprises me that she let it go at that. She was usually so nosy...) I was gone for about a week, week and a half.
Another time, just after my Mom died, I took a week off on Family Leave. After that, I was still a mess, and my doc had me off for about 3 months, then part time for another 3. They filed it under short term disability, *I think* during the months I was gone.
How did I know I needed to be off? I was crying at my desk every time someone asked me how I was, I was showing a lot of exhaustion and other physical signs of mental stress, and I was taking sick days a lot just to keep myself going.
Hope this helps!