Spousal rape is defined at section 262 of the Penal Code in California. The section is too long to quote here so read it at http://www.leginfo.ca.gov/cgi-bin/displaycode?section=pen&group=00001-01000&file=261-269. Note that the punishment is set forth at PC § 264 is 3, 6 or 8 years in the state prison assuming you cannot get probation as set forth in PC § 262.
Excuse me I didn't answer your question. Pursuant to PC § 805, the statue of limitations is based on the maximum you could get under the statute. For a crime punishable by 8 years the SOL is 6 years. PC § 800 defines the SOL for such crimes as 6 years.