atmega2560 set timer 2 to wake after x minutes ("power save" mode)


I want the atmega 2560 to enter in "power save" mode, leaving the timer2 running, and in this timer(overflow interrupt routine) increment a variable that determines whether the atmega should return to "sleep" or wake up.

1.) I need some help to figure out how much time will it take for the timer2 to do an overflow.

2.) By the way, is this the correct instruction to enter the power save mode?

SMCR=0b00000111;  //power save