It will vary depending on a number of variables.
1: Your airbrush. A "gravity feed" brush will typically require less pressure than a "siphon feed" (though in reality, they're all siphon feed).
2: The medium that you're airbrushing. The thinner the medium, the less pressure you will need.
3: The distance from your airbrush to the surface you're painting - the closer you are, the less perssure you will need.
4: What you are trying to achieve. Are you applying an overall base coat or are you detail painting? Are you applying a camo pattern? Your pressure will vary depending on what effect you're trying to achieve.