In sleep mode, the 12F and 16F PICs draw ~2ua, so you could keep them alive for months. If all you have to to do is wake up ~ once per second, do something that takes 1msec (increment your time counter) , and then go back to sleep, it would still only be an average power draw of <<1mA. Another approach is have a 32kHz watch crystal oscillator as your instruction clock, in which case the power draw is also reduced by a factor of 100 to 1000 compared to clocking at 8MHz.