At what % battery level will the Phantom land if app battery timer isn't enabled?

If the "* Battery Limited" selection is made in the Maximum Flight Duration setting then the aircraft will come back either when the battery percentage reaches the automatic return value set in the DJI Go app or when the aircraft detects that it has just barely enough power to make it home.

We have tested with the Battery Limited version of auto return for the majority of our field testing with no issues. As long as your battery is properly calibrated the drones idea of when it needs to head for home should be pretty accurate.