Jeez, I use half used AA Alkalines frequently, and my rule of thumb is that if they're lower than 1.37 volts when I'm setting up that day, I won't try to get the opening act AND the main act on that set, unless I'm desperate.
With a fresh set of Alkalines, I usually get at least two hours without dropping a bar. Note that I only run line in or internal mics. I don't run plug in power. I usually keep the backlight on when the deck is running.