The period is low despite the far greated recording capability with solid state units because of pressure from the unions who would rather conspire against safety and let a few more of their members and hundreds of the public die than show up the incompetence of live members
While longer time would rarely aid accident investigations it would make incident investigations far better and prevent more accidents (and deaths) in the first place.
It is time that routine CVR monitoring was introduced to catch rogue pilots before they kill, encourage compliance by all pilots and help fault find faulty recorders.