It has already been said, pressure depends on a lot of things. It's good to remember that your airpressure is also a tool you can use to get specific effects. I myself am constantlyadjusting the presure while airbrushing.
High pressure (2,5+ bar):
-Thick paint (opaques when you want to flood fill an area)
-Long distance from work (again generaly when floodfilling)
-Fast moves (doing long hairs in one stroke)
Medium (normal) pressure (1,5-2,5 bar)
-Basicly all the work you do while your not verry close or verry far away from your work with normaly reduced paint.
Low pressure (less than 1,5 bar, for me normaly under 1)
-When you generaly need more controll (thin lines, fine detail, soft shadows, slow moves)
-Over reduced paint (higher pressure will than cause spiders)
-Verry close on your work (again to avoid spiders)
Beware that what is high or low pressure also depends on your airbrush, the above is generaly the case when I use 0,15-0,2 nozzles. Bigger nozzles might requier more pressure.