The LED will be brighter with a continuous DC forward current more than 30mA but might burn out at any time. You calculate the value of the series resistor so that the current does not exceed 30mA, but the calculation is difficult unless you know the exact forward voltage of the LED by measuring it at the current you wish to use. The current and brightness will decrease as the battery runs down so to keep it bright, a constant current circuit could be used instead of a series resistor.