I bought an ATMega328p chip a while back to try and make a barebones board but couldn't get it working. Also, the price of the parts was far more than buying a Nano or Pro Mini so it seemed a little pointless.
I have seen some people running strip down boards off button cells for a very long time, usually only transmitting something occasionally though.
I might try a 433 MHz transmitter on a temp sensor setup to see how well that works.
http://jeelabs.org/tag/lowpower/