Depends on the paint. Parma recommends somewhere around 60 psi for the Faskolor paints. Pactra says you can shoot their Racing Finishes in the neighborhood of 20 psi. The thinner the paint, the less pressure you'll have to use...
Roughly 15-20 for solvents like Spaz-stix, and 30-40 for acrylics like FasKolors, although you'll find some spray easier than others. Thin the FasKolors with a teaspoon of water if needed.