Depends on the paint. Even within the same brand and line (I usually use ModelMaster enamels), I've had some samples that were perfect for airbrushing right from the bottle, but then a another bottle of the same color would need fifty to hundred percent thinner added. The usual rule of thumb is to hit the consistency of 2% milk. Until you're experienced enough to easily judge this, you'll be doing a lot of trial and error.
Best thing to do is to practice first using a plastic milk jug or a spare sheet of styrene. If the pain won't come out or the surface is pebbly, you need more thinner. If it is runny then you probably need more paint. Usually when its right, the paint flows well and the surface being painted will hold a gloss sheen for a few seconds before flattening.