With 4AA are only good for 72 hrs with backlight off in the 187/189.
I don't know what voltage the battery warning comes on at but I just measured the consumption, it uses:
~13mA without the backlight, that should be about 150 hours with a 2000mAh battery.
~43mA with half-power backlight, about 46 hours with a 2000mAh battery.
~80mA with full-power backlight, about 25 hours....
It's definitely not the most contrasty meter in the world and the screen background is quite a dark color so I'd probably want the backlight on if I was using it all day.
The annoying thing is that there's two levels of brightness so you can't turn the light on/off with a single press to save power You have to press it multiple times.
(and the second level of brightness isn't
that much brighter.
)
Bottom line: You'd be changing the batteries once a week if you were using it all day. It only takes about ten seconds to do, so...