According to the Brewers Associations Draught Beer Quality Manual, for jockey boxes, "Set CO2 pressure to 35 to 40 psi on 120 ft. coils. Shorter coils are not recommended, but if used, should dispense at 30 – 35 psi."
I have a 3 tap jockey box I built with 50 ft. coils--I only used it once so far and the pressure was definitely too low at 15 psi. I'm using it again in 2 weeks and plan to follow the recommendations above then.