Depends on the enamels Paul. I spray Testor's Model Master and Humbrol enamels. (don't like the acrylics!) I use the Testor's thinner for enamels. I usually mix it paint to thinner 60/40 to 50/50. Seems to work well and I keep the air pressure moderate as I use a valve at the airbrush to regulate my air flow.
I also tend to spray a bit closer than some folks only a couple of inches (hey it ain't a spray can!)
Some paints like Alclad metalizers and primers are mix and spray straight from the bottle.
When I spray acrylics (usually Vallejo these days) they are thinned a lot with just plain water as I use them for weathering, light effects, and such over the enamel base colors. The water drys slower but I can also use the air from the airbrush with paint off to dry it faster as long as it isn't puddled.