"Would it be when there is no ligh at at the end of the pen"
When using your DomPen, the light on the bottom of the pen may still come on even after the oil in the cartridge is finished.
At the top of the DomPen, you may notice three small circles that look like windows. These windows display the oil present in the cartridge. When you are able to see clearly through the top two windows, your DomPen is near finished.
Please be mindful when inhaling as the oil gets lower. Taking big puffs may result in a harsh taste as less oil will heat the metal in the pen.
I hope this information helps! 🙂