Generally different for each line of paint.
Lots of factors go into it. How much air pressure you are running, size of the needle in your airbrush, the type of effect you are looking for.
For me and the paints I use, or have used.
Tamiya: 50/50 paint to thinner (using Tamiya's own thinner)
Vallejo: Model Color: 3 parts Vallejo AB Thinner to 1 part Vallejo Model Color paint.
Model Air: Should be good to go out of the bottle, but Vallejo recommends a couple of drops in your paint cup.
Testors Enamel: 50/50 with enamel thinner
Lifecolor: I have used, but only once, and don't really remember the thinning ration, seem to remember that 50/50 was too much using their thinner.
Humbrol Enamel: 50/50 with enamel thinner
Gunze Acrylics: 50/50 with acrylic thinner
That covers the basics, and for the lines I have used for general use at about average pressure (15-20psi). Finer spray control you need to drop your air pressure more and thin your paint out a bit more.
And of course, practice on an old model or piece of scrap to try out your own recipes.
