I had out of state plates for a year before I could pass smog. The stickers on the plates were EXPIRED for 5 months of that time. I had paid registration in Cali, but couldn't get the stickers until I passed smog. Nobody bothered me. Cops didn't even notice because they don't know what color stickers a MD plate is supposed to have. I probably could have kept them forever. Car was daily driven.
Sorry, Cali. I paid every penny on registration, though, but you wouldn't have known from my plates.