Hi,
The are way's to improve the battery live.
jackorocko estimation's applies for a continuous usage. But, depending of your application you can:
-Wake-up your circuit at fixed interval with a timer to have maybe few % of active current (discontinuous usage).
-Add a large (low leakage current) capacitor to limit battery peaks and so increase is life.
(this is almost the trick applied on some TV remote control where they are IR led currents peaks)
-Reduce the atmega clock as low as possible for your application (32Khz can be already enough)
Sure the battery life will not become years, but it's can help.
Olivier