Norman Vincent Peale wrote The Power of Positive thinking which describes the placebo effect but I don't know if studies have been done to determine for exactly how long?
Maybe if you stay positive the effect can continue indefinitely?
Now, you could write a book entitled, The Power of Negative Thinking. The subtitle could be, Misery Loves Company.
That placebo effect is an interesting thing. It could have been as simple as stamping the Denmark flag on our placebo that would have gotten us to approval.