The battery life with 4 new alkaline batteries is a solid 12 hours, powering LEDs and electronics. I ran them all the way down to the point they stopped working (no LEDs, no sound). They worked great right up until they stopped working. No distortion, no loss, just dead.
The box was designed to make it easy to replace the batteries - pull it out of the holster, open both sides, pop out the 9v's and put in 4 new ones. Probably less than 60 seconds.
Here's the cool part: I play in church and so they died halfway through the service. I didn't realize they were dead until I strapped on to play the special, so not enough time to replace the batteries. No problem. Pulled the 1/4 jack from the battery pack and plugged it directly into the bass. Now the internal batteries are powering the electronics (no LEDs, but better than no bass for the special). Worked great and carried me through the service.
Essentially, the internal batteries become the emergency back up for the external battery pack. No more worrying about battery failure.