Well, according to these web sites...
http://www.watertips.com/info/FAQvalv.htm#wiresize
http://www.watertips.com/instruct/RBwire.htm
...it would seem that a single 18ga wire will work for distances from 600 to 3000 feet depending upon the pressure in your system. Now I must admit, I don't understand the huge difference (600 v 3000) just because water pressure doubled, and that information is based on Rainbird valves (the Hunter valves are a little different).
One other thing to keep in mind, the numbers listed is for total wire length. So that's from the controller, to the valve, and back. So when the chart indicates 18ga wire is good to 600 feet, that means the controller must be within 300 wire feet of the valve (300 feet of control wire from controller to valve, plus another 300 feet of common wire from valve back to controller). Additionally, these numbers assume only one valve is operating at a time.
So it looks like safe distances (controller to valve) would be the following:
18ga: 300 feet
16ga: 365 feet
14ga: 430 feet
12ga: 480 feet
If you double up two 18ga wires, that would be about equivilent to one 15ga wire. So based on the numbers quoted above, two 18ga wires should be good to about 400 feet.
If the chart @ 80psi can be believed, then two 18ga wires should be good upto about one-thrid of a mile.