Severe (or more) turbulence... how common?

Flying from Palo Alto to Santa Monica yesterday, we hit (I think) a rotor
coming over the mountains north of LA. We'd had some fairly strong
up and downdrafts, though nothing really unpleasant, when suddenly
we were going DOWN - about 2000' in 10 seconds or so, everything
stuck to the roof for maybe 5-10 seconds. It was an interesting ride...
most things landed shortly afterwards (and I even found a flashlight that
had disappeared) although my E6B remained lodged somewhere
and fell on my head on final.

Afterwards, my wife asked me how often this happens... once per
flying lifetime, much less, every few years. I couldn't answer... for sure
the first time in my 600 hours, but that's not much.

So here's my informal sample... how many people here have had a
similar experience? Anyone care to guess how unlikely this it?

(And to forestall, or try to, the people who tell me I should know something
about mountain flying, yes I HAVE read Sparky Iverson's book, and
anyway living in the Bay Area it's hard to go anywhere much without
overflying mountains. I was over 2000' above terrain too, 9500'
over a ridgeline at about 6000').

John