It depends who you ask. Personally, I think it is very rare. There are some people who will loosely define DP as a detachment from reality, an out of body experience, where things just don't seem real. Every person who has ever lived has probably felt like that at some time or another, but it is a fleeting experience, not troublesome, and quickly forgotten. Why would you even worry about that? It's like thinking you have a cold because you cough once. I think defining that as DP is a bunch of malarchy.
I think in order for someone to have depersonaliztion.
A. It is troublesome.
B. It is either recurring and episodic, or chronic.
C. It affects some other aspect in their life, i.e., it can create depression, anxiety disorders, decline in cognitive function, social withdrawal, etc.
I can't speak for everyone here, but my first episode of DP/DR, I knew something was dramatically off, and I freaked out. It was like no other feeling I've felt before, and it was a feeling that would warrant a title all its own. I later learned it was called depersonalization and derealization. I would say the number of people who ever feel that is very small.
I remember seeing my old girlfriend after she woke up one time, she said she had just had this short out-of-body experience and thought it was cool and laughed it off. That doesn't count.
There are also people who are just kind of existentialist by nature. They question the reality of everything. I am pretty skeptical about that as well.
So in answer to your question and in my personal opinion, very rare.
