About "reasonable flying time" - are there any options available to use solar panels? Most importantly are solar panels good enough to power the unit?
A 15W solar panel is about three square feet, and probably weighs several pounds. A 15W quadcoptor is about two square feet and has a takeoff weight of less then three pounds.
You might be able to carry a small solar panel as a backup in case the copter runs out of power in a remote location, but it would have to sit for quite a while before it's ready to fly again.