Don’t know yet, I updated the firmware of the node on 25 juli 2017 with a firmware containing the great Battery ADC method from @Charles. And still haven’t lost 0.1V from that day (the resistor battery monitor was unstable).
The ArduinoMiniPro has a brownout of 1.8V. So I hope it will have enough power for atleast a year 
My first prototype had the default brown-out of 3.3V, wasn’t very low power (had 1 low current power indicator led) and used 3 very cheap AAA batteries as a power source. Giving enough power for just 2 months before going down.