The exact time between first warning and before RTH would likely depend on battery type or chemistry and quality. It is intended to give you ample warning so you can land or complete the flight.
But as you mentioned, RTH would follow when appropriate so no need to panic unless you are in a bad place for RTH to occur.
But as you mentioned, RTH would follow when appropriate so no need to panic unless you are in a bad place for RTH to occur.