I reside in CA. I had a restraining order placed against me for 6
months (now expired).  How long is this information visible to the
public or via a backdround search?  Does it drop off once its expired?

I am not an expert on this exact issue, of course. However, assuming
it is a domestic violence sort of restraining order, I can tell you it
stays visible to at least the police for a very, very long time. Such
things cause one to be prohibited from owning/possessing firearms, and
I know that goes on for a very long time. A few years ago, a number of
law enforcment officers were romoved from their employment because of
dv issues some considerable years earlier, and they could no longer
have firearms.

This is clearly an area where you need to see a local attorney, in my
opinion, since having a firearm when prohibited can be a felony, even
if you are unaware of the prohibition.  Internet answers in this
specific case can be very risky.

At the time of expiration, if not renewed, it does expire.  I have
seen restraining orders for as short a time as a year, and as long as
10 years, so the answer is: However long the judge set it for.

I didn't read that very well..., disregard...