From the HyperPhysics site on Quarks:
It is postulated that it may actually increase with distance at the rate of about 1 GeV per fermi. A free quark is not observed because by the time the separation is on an observable scale, the energy is far above the pair production energy for quark-antiquark pairs. For the U and D quarks the masses are 10s of MeV so pair production would occur for distances much less than a fermi. You would expect a lot of mesons (quark-antiquark pairs) in very high energy collision experiments and that is what is observed.
From what I can gather, the actual increase in energy per fermi is not known to a high accuracy because the range is so incredibly small. Considering that the strong nuclear force is only able to participate in interactions at distances of less than $10^{-15}$ m it is understandable that no exact numbers exist for the maximum separation two quarks can have.