[metalalchemist]

That depends a lot on the paint and what medium you are using.
If you are using an airbrush, you want to go with milk consistency.
If you are using a regular brush, you are looking for something not as as watery that it won´t stick to the figure and not as thick that it will leave the brush marks.
For example with testors acryl. I normally use it as it is out of the bottle, with regular more dense cheap acrylics (ie: craft paints), I use like 2 drops of water per 3 of paint.
Just remember, when you paint, if the color is going too thin, don´t worry, just give it more layers of paints. It´s better this way (longer, I know) but the end result is incredibly smooth and nice.