As noted, if you lose all engines, your max flight time is pretty limited. At cruise altitudes, you've got roughly 100 miles max range, and maybe 25 minutes max flight time available. Hence (assuming a healthy battery), 30 minutes on the battery is enough to keep basic instruments as long as you're going to be airborne.
The hydraulic motor generator is unrelated to the all engine-out event. It's back up electrical generation to cover the case of a failed engine generator but with the engine still operating.
Some aircraft (the 777 comes to mind) have the RAT drive both a hydraulic pump and an electrical generator. But the 767 is not so equipped.