- I would power A0 (ADC)through a digital pin. This way it reads voltage only when needed, not always connected to battery and draining power.
- Besides powering the nrf radio through a pin (transistor) is a good idea as well.
- Putting the mcu To sleep sure will add few more days to the life of the battery.
Battery in correct connections should live more than couple of years, I had ultrasonic distance reader project connected with nrf and pro mini. That lived months without a noticed drop in battery