7200 milliampere-hour divided by 400 milliampere-hour (dr-680 current draw) = 18 hours of run time in theory. In reality you'd want to multiply that number by .85 since you don't want to run the battery down to empty so you're still looking at at least 15 hours from it.
I'd be a little wary of these numbers. You're math is right (I'm willing to concede anyway, but isn't the 680 more like a 750ma draw?), but I'm afraid there might be some marketing hype in their reported specs. One part says 26 watt-hours and 12volts, the other just says 7200mah.
26watt-hrs = 26000 mW-hrs, divided by 7200mah = 3.6v, which is suspiciously close to what a single li-ion cell provides. Alternatively, 26000 mW-hr divided by 12v = 2167 mah, not 7200mah.
I'm wondering if the 2167mah at 12v is really what the battery has, and the 7200mah spec is just marketing BS, meaning the battery internally uses 7200mah of 3.6v/3.7v li-ion cells, that then need to be stepped up to 12v using a step up circuit. If this is the case, the battery may only provide 2167mah at 12v, or about 5 hours of use.