Mike is right, there is no pat answer because it depends upon how contaminated the car is. In the below picture, the contaminants you see on the clay came of of HALF the ROOF, which is kind of small as far as roofs go...
1954 Ford F-100 - Extreme Makeover - Process and products used
This truck was re-painted approximately one year ago but just from exposure to the outdoors has made the paint vulnerable to air-borne contaminants...
This shot was taken with my trusty, dusty Canon Rebel with the flash on after claying the roof... 
Without the flash on here's what the clay looks like and this is what it looked like in person... The roof and the other horizontal panels pretty much felt like sandpaper before we started.
Ewww.... 
Besides restoring gloss, by removing the film of contaminants off the paint you'll make machine polishing easier, safer and more effective and enable your choice of wax or paint sealant to better be able to bond to the paint.
After claying and machine polishing and machine waxing... 
I'm pretty sure I answer that question along with a lot of other frequent questions about clay and claying in these videos...
How to use detailing clay to remove "Above Surface Bonded Contaminants" and restore a smooth as silk surface.
Part 1 - How To Use Detailing Clay
Part 2 - How To Use Detailing Clay
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