karlok:
Can it be approximated with 2000mAh/0.82mA = 2440h (even if the relation is not really linear how the graph shows)
Yes, but I'm not sure where you're going.
If you mean that you intend to NOT switch the battery test circuit on and off, but just leave it connected all the time, then This is giving you a rough idea how long the thing will last. But in this case you are really measuring the open circuit (no load) voltage of the batteries, and in this case there's no need to draw so much current from them. You could set up a voltage divider with 100K on each leg, and the current would then be only 45 microamps, and your battery life would hardly be effected beyond normal shelf life at all, maybe not at all.
This was what I was getting at back here: rechargable batteries for uninterruptible power supply - #44 by gardner - Project Guidance - Arduino Forum
But then Jack mentioned that you would be better to test the batteries under load: rechargable batteries for uninterruptible power supply - #45 by JChristensen - Project Guidance - Arduino Forum
So now we're here. If you want to just measure open circuit voltage on the batteries, then no need for anything fancy, just a pair of high value resistors is all you need.