From what I've read it could be as little as 3 weeks or never.  I just found
out last week that I have HSV.  I never had any symptoms until several
months ago.  They were not the normal symptoms.  My doc sent me to a
dermatologist for further treatment.  After 3 visits to my doc I finally got
him to test me.  I am almost sure I got it from my last girlfriend, who
showed no "normal" active signs.  We are still waiting for her blood tests
now since although she has been to her gyno twice for std checks, she was
never given any blood tests.  DUH.
If she is negative then I have had it for who knows how long.  It could have
been years.

Initial incubation can be two or three days to three weeks.  But
it's entirely possible that it can go longer.  You could pick up the
virus and have no reaction at all.  Then, as your body ages, etc.,
or just because, the virus could suddenly decide to go postal, so to
speak.

It's also possible at some point to have had such a mild primary as
to go unnoticed, or to have been mistaken for something else.

None of the above scenarios are particularly rare.  HSV is a very
variable disease.

Mike