If it's not FAA policy, it should be: No one is left alone in the cockpit at any time. Either a jumpseating pilot or flight attendant be in the cockpit. If it's a 50-seat CRJ with one FA, the pilot should not be allowed to leave the cockpit unless there is a non-revving crewmember who can be pressed into service. 50-seat RJ flights aren't usually very long. As my dad said many times "You better go before we leave because we're not stopping."