Watchdog Timer for atmega 128
There is no way you can make the Watchdog timer longer on Atmega 128. It is max 1.8 sec if you set all 3 bits of Timer Prescaler.

For Atmega 328 the max watchdog time is 8 sec and longer time can be achieved using  watchdog interrupt and count up (or down) until you want a reset and then force the reset by setting the shortest watchdog timer.

Atmega 128 doesn't support watchdog interrupt, so the trick with interrupt can't be used.