perhof:
You will have to use a watchdog timer to put the ATmega to sleep.
The timer isn't needed to put it to sleep, it's needed to wake it up after a delay. Although the timer will only time intervals up to 8 seconds, you can repeat the 8-second sleep as many times as necessary to achieve the overall interval. There are various sleep/power management libraries which can take care of this process for you, and deal with keeping the clock up to date so that millis() behaves correctly, and powering down other on-board devices to reduce the power consumption while the processor sleeps.