Hi All,
V3 run time should be the same on a 6volt or a 12volt. The only difference is that it will run hotter (not problematic) if using a 12volt. As previously mentioned, the 12volt setting is really just a battery protection. Folks in the film industry use a lot of 12v equipment and often prefer to have all devices operating on the same voltage.
Personally, I have always had a my V2/V3's at 6volts even though I hit them with a 12volt. This way, you can run either. The reverse is not true.
One exception on batteries in the Eco-Charge EC-90 which utilizes a really cool voltage regulator that does not burn up the excess voltage in heat. People using the EC-90 with a V3/V2/DA-P1 or other 7.5 or 6volt device will actually get nearly double the run time from them as they would with a 6volt or standard 12v battery.
I hope this helps.
Marc