This depends on the wire resistance (and in turn affects the voltage drop as Shawn mentioned). Generally thicker wires work better as they have less resistance. If you know your wire’s gauge, you can use the calculator here to compute the resistance:
http://www.cirris.com/learning-center/calculators/133-wire-resistance-calculator-table
Assuming the valves need 0.2A holding current to operate, the resistance times 0.2A gives the voltage drop. Keep in mind that if the voltage drop is too steep, you can always switch the power adapter to something with higher voltage (like 12VDC adapter). OpenSprinkler DC is fairly flexible in terms of the input voltage and 12VDC adapters are pretty easy to find.