California Driver Handbook defines speed limit as "The maximum speed limit on most California highways is 65 mph. You may drive 70 mph where posted. Unless otherwise posted, the maximum speed limit is 55 mph on a two-lane undivided highway and for vehicles towing trailers."
I assume that the zone starts right after the speed limit sign post. What is not obvious from this definition is where the speed limit zone ends.
Case in point: Highway 101 in Los Angeles urban area generally has a maximum speed limit of 65mph. It has occasional 55mph speed limit signs, such as this one, where 101 approaches Hollywood Blvd from the south. Where is the exact end of this 55mph speed limit zone?