How much time before airbrush is clogged?

Depends on what kind of painting I'm doing.  If I'm spraying very light coats to build up, I generally wait about a half hour between coats.  In that time, I don't mind letting the paint sit in the brush.

If I'm doing heavier top coats, I'll wait a day between.  Then I clean the brush right away.  It's easier to quickly clean out the brush than to breakdown the entire brush and have to scrape paint and soak needles and nozzles.

-Fred

I usually only wait as long as needed for the paint to be dry to the touch before giving a second coat.  Living in Southern California, that is usually not a long wait, perhaps as little as five or ten minutes.  I usually don't completely clean my airbrush between coats, although I may shoot a bit of thinner thinner through it to head off any clogging issues.

If I'm spraying mist coats,   I allow 10-15 minutes.  Wet coats - 25-30 minutes.  Those figures are for enamels. 

If I'm spraying lacquers,  I cut those times in half. 

I don't bother 'thoroughly' cleaning the airbrush while waiting to apply the next coat.  I just run some thinner through the a/b until it sprays clear.  Haven't experienced any clogging problems (yet) using this method.