Recent changes to the PostNote design achieves close to 1.5 months of operation. 

The Arduino Pro Mini microcontroller now goes into sleep mode if light is not detected by an LDR. Once asleep, the watchdog timer wakes the microcontroller up every 8 seconds. If the LDR detects no light, then the microcontroller goes back to sleep and the wake/sleep cycle continues. If light is detected, the main PostNote sketch is executed. 

Adding the TP4056 battery charger module ensures that the 18650 batteries don't discharge below minimum voltage this time. This reduces the amount of total energy that can be draw from the batteries, but it also means the batteries don't get damaged and can be recharged.