After reading many post of people getting months of ESP8266 running time off batteries I decided to set up my own to see why my battery life was terrible:

Parts:
Node MCU ESP8266 (CH349G Serial Chip, AMS1117 Voltage Regulator)
LiPo Battery: 2S 850mAh (7.4V)
ADS1115 ADC (to measure voltage, 500K Voltage divider)

Test setup:
Two ESP8266 setups were completed, one ESP was standard and the other had the LED and Serial Chip disconnected to conserve Battery.

Test Program:
ESP Wake every 20 seconds (with radio disabled)
Take voltage reading and store in RTC memory
Deep Sleep
……………………………………………………………..
Every 5 minutes (15 wake cycles)
Take voltage reading
Connect to network and transmit all data to Influx Database.
Disconnect from network
Deep Sleep

Results:
You can see from the below screenshot the battery voltage over the duration of the test:

Unmodified ESP8266:
Time from 8.36V to 7.28V (97% to 7% of Li-Po capacity) was 87hrs and 20mins (3.6 Days)

Modified ESP8266: (No LED or Serial Chip)
Time from 8.36V to 7.28V (97% to 7% of Li-Po capacity) was 101hrs and 16mins (4.2 Days)

Conclusion:
Months of usage seem far from achievable with a minimal setup and all precautions taken. Actually the ESP seems pretty unusable on a battery for anything more than a measurement every few hours.

Further Improvements:
The stock voltage regulator is a known power drain, an alternative is recommended but I did not get around to that yet.

That’s it!