ATmega168P
Low Power Consumption at 1 MHz, 1.8V, 25°C for ATmega48P/88P/168P:
–Active Mode: 0.3 mA
–Power-down Mode: 0.1 µA
–Power-save Mode: 0.8 µA (Including 32 kHz RTC)
.3mA in active mode and at a 2000mAH battery ( those small lithium ones) - thats 6,666 hours or 9 months.