Several years ago I built a low power end device using the @ch2i arduino pro mini based design. I snapped in a LiFeP04 cell as it shipped from China, never charged.
It has reported BMP280 values every 15 minutes totalling almost 102,000 transmits before the single cell dropped below 3.0v today. I really wanted to keep this stacking up the frame counter but with a change to V3 stack in the near future I thought a new cell would be no harm!
Quite an impressive low power result and I would suggest with the smallest of solar harvesting circuits would have been a deploy for life device?