I've read some books about depression and some things made sense to me and some things didn't. One complication of generalizing is, of course, that everyone has a different DNA/nature/nurture.
One book said something that made sense to me: Like Ricki mentioned, a traumatic event can lead, unfortunately, to reliving or "CHURNING" a depressing line of thought over and over. I suppose it's a form of PTS, post-traumatic stress. For me, the trauma of not being female was not anywhere as traumatic as transition. I underestimated how society's bigotry would affect me. I felt I was strong and was around educated open-minded people. While I never experienced open bigotry like many TS's have, I did lose my job ("lack of work" was the culprit, my boss said. I knew the real reason but it was never spoken).
I think if you find two things in your NEW LIFE, decent work and a loving spouse, that is the best medicine for depression. I lack both but luckily will be able to draw on my pension/medical plan in 5 years.
I never found anti-depressants to be of any use to me (I took them for 6 months) because the problems that bothered me (lack of work and a loving spouse) remained. I also noted that, in the contraindications for many supposid "anti-depression" drugs, "suicide" is one possible side-effect (for a minor percentage of the people taking the drug). That didn't sound good. The thing that MOST helped me was finding a thing called COGNITIVE REASONING. Kaiser Permanente uses that kind of rational reasoning in its anti-depression groups to combat sad feelings.
Another thing that helps is just being in an anti-depression group. We went around the room and it was surprising to me how some people could get very depressed over things that, in my world, seemed like nothing important. That lead me to realize the obvious: We create our own hell by self-torment and that the best thing to do is STOP thinking about depression. Take a walk, read a book, meet with friends -- do anything to stop pondering and destructive CHURNING. I've noticed, for example, that if I begin thinking about suicide, I immediately get VERY depressed. Thinking about ways, thinking about after-affects, it's all poison. The worst thing is visualization.
A person looking at me during this period would ask, as the anti-depression group would, why put myself through trauma? Isn't life difficult enough without enabling self-inflicted trauma? And the answer, of course, is "YES."
You asked, "how long can people be depressed?" The answer is, in my opinion, however long YOU decide to be depressed. You're the captain of your own course. You can ignore cesspools or cruise your boat right into them.
There is much truth in the simplistic saying, "you're as happy as you want to be." The next time you start to get depressed, try NOT thinking about it. By not letting it take control, doctors say that it can, over time, help break the CYCLE of that unfortunate tendancy of your mind WANTING to CHURN.
Ricki said, "I literally allowed myself to get depressed." I did, too. Distract yourself. It has worked for me.
Hugs, Teri Anne