California and federal law allows firearms to be directly transfered between familial parties when a death is involved, inter- or intra-state. Inheritance is one of the few exceptions where guns may be privately transferred across state lines without the involvement of an FFL. If your mother is the executor of your father's estate, she should be able to give any of his guns directly to you. I don't believe there is a time window to do so. You don't need any HSC or Oplaw forms since you are taking the firearms out of California. If you were a California resident receiving firearms from another state or within the state, an HSC and Oplaw form would be required for any handguns inherited.
See threads here for more detailed info (Librarian's posts in particular):
If you're worried about the (hand)guns you already possess not being registered to you in California, there is no requirement that they be. Some crimes committed with an unregistered handgun carry a harsher sentence than those committed with a registered one, but that's all.