Like others, I use about a 1:1 ratio, paint to thinner. The drying time depends on weather conditions, cold or humid it takes as long as 3 or 4 days.
While I buiid up several coats, I often do that in one session. I gather all parts that will be airbrushed black. I put a thin coat on first on all parts. Then I go back and in order give them a second, heavier coat. Then a final coat. This last coat should not go on thin. The secret to gloss is to put it on heavy, to the point that it is within seconds of running. It is a game of brinkmanship. How do you know it is about to run? Experience and a good positionable light. You must be able to see light reflected in paint. That is the best way to judge the gloss/thickness. If it didn't run on you, it is not too thick.
Some folks make a heated dryer (some folks make them from food dryers). I am afraid I will warp plastic. Lately I have been using a modified storage case (what you transport models in) as a ventilated dryer. Enclosing model/parts in a dryer eliminates dust while drying- a problem with slow drying paints.