Think of it more like the gas gauge in a car. Some of them stick near the full line and then drop comparatively faster once they start to drop. Others drop fast and then seen to show more milage from the bottom half compared to the top half. My gas gauge is in the later category. I often get to half tank on the gauge with only about 100 miles driven and then get 300+ miles from the gauge's half mark.
Darwin batts are more like the top heavy gauge. It will read full for a long time and then drop faster. So if your batt gauge shows half on the screen, you may be closer to safety cut-off than you might logically think. The gauge is not a directly accurate indicator. Just remember, when the gauge starts to show some depletion - start paying attention to your needs for the remainder of the day.
It IS an amazing batt with amazing charge cycle. How long it lasts is a function of many variables. But, mostly it is the Amps drain. Amp drain will be determined by the attys/carto Ωs you are using combined with the watts setting and, of course, how much you vape. After you have used it for a while you start to get a sense for what the gauge is telling you relative to
your use patterns so you will start to get a better feel for if you want to top off charge at some point during your day (like if you are going out on the town and may not have access to a way to charge) or if you want to let it slide (if you know you wil be home where you have access to charging).
Hope this helps you.