There are too many variables to give any kind of reasonable answer. The variables would include (but are not necessarily limited to):
* battery capacity (e.g. NiMH "milliamp-hours")
* battery manufacturer (not all are equal)
* scanner activity (% of time it's "playing voice")
* scanner options active (backlight, LED, audio volume, etc.)
If your primarily going Battery Power, go re-chargeable. I can go through a set of 4 "standard' batteries within a couple weeks. I don't think I use it a lot. Non-rechargeable high grade about twice that long. Once power starts waning however, the drop off is very quick (when the low battery alert comes on).