How long can the iPhone 3GS record video before the battery runs out?

My father invested in an iPhone 3GS Tripod adapter for his tripod, for some conferences and lectures. He has a 16GB 3GS, which will record about 8.6 hours, according to another thread I've read about on here. But he doesn't really want to bring the USB/Adapter cable with him, and have to worry about finding a power outlet. Assuming his phone is on full charge, and bluetooth/wifi are shut off, and brightness is kept a bit dim (not too dim) - can anyone give me a round about answer of how long the it will record video before the battery dies? This function is today, so timing is critical. I will have him take the charger/usb with extension cable just in case. I'd appreciate anyones input