With out of state plates, how long can I park on the street?

Excelsior doesn't require parking permits like a lot of other SF neighborhoods, so as long you aren't parked when they do street cleaning you should be okay.

I assume you will have to get California State plates when you renew your registration--I don't know if there is some law that forces you to do that beforehand. Goog luck and welcome to CA.

Depends on which street and what the signs say.