As jon-slider says the light is going to become very dim when the battery voltage is low.
You can look at TA’s xp-l2 test data and look at the vf (blue) vs Output (orange).
This is the actual voltage to the led. A battery will have voltage sag under a current load.
This is only a example, leds have different vf’’s at the same current but they are typically close.
At a loaded battery voltage of 2.5v your going to be almost in the dark. Once the load is removed the battery voltage will bounce back.
You should be able to recognize the battery is getting low visually way before the battery is at 2.5v loaded.
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This does depend on what mode your are in though. If your using the light in low mode and only drawing a few ma’s then your probably not going to notice much change in output until the battery is below 2.5v loaded. Best to check voltages but using in a higher mode will let you know once you have done it a few times.