Answer
The absolute minimum the Power On Reset shall be asserted is 150ms after the power supply is stable and the WL_REG_ON and/or BT_REG_ON signal are asserted.
However we recommend waiting 250 milliseconds to ensure the internal reset sequence has completed.
Note: The signals WL_REG_ON and BT_REG_ON are pulled high by default internally for the Wi-Fi and Bluetooth radios to be enabled when power is applied.