Soon as you can afford it, get a Hobbico VoltWatch, or a expanded scale voltmeter, and one day test those batteries after each flight to see exactly how long YOUR batteries last. Average numbers won't help you if you have a bad cell or whatever. If you can't do that, find a friendly experienced flyer at the field, and have them test your battery (it's easy). It's a drag to lose a plane because you didn't have the equipment to test the batteries.