Ugly Preamp or Ugly Battery Box? Two different animals. The battery box will run much, much longer than the preamp will, since there is no amplification circuit being powered.
With the Ugly Preamps, I recharge the 9.6V NiMH batts I use in them before each show. For a festival I could probably go all day, but since there is no low battery indicator on the preamp I change the battery half-way through the day just to be safe. If you want to measure the battery with a voltmeter to check, make sure to do so while it is powering the preamp (or batterybox) under load. Don't take the battery out to measure it because you will get a false reading.