How often should I paint the exterior of my house?
There are just too many components to give you a definitive answer.  Atlanta doesn't have extremely harsh conditions, so if your house had perfect surroundings the paint could last 50-100 years.

Here are some factors:


How well was the house painted.  You ask if you should do a second coat.  Yes.  Basically if you have any paint "holes" were the first coat didn't fill this allows water to get in.  The water can expand during freezes (I know this is like 7 days a year there) but more importantly the water will sit there during warmer weather.  Do you need a thick second coat?  No.  But I would always do one on my house for non-"ceiling" areas.
What is the architecture of the house.  I have seen many many wood siding houses that have a flat cross rail.  Well guess what the water sits in these places, kills the paint job, rots the wood, and spreads.
Do you have anything else surrounding your house that hurts the paint or induces more moisture?  So ivy, big trees, large bushes, whatever.  I have big oak trees that drop limbs and acorns.  No big deal right?  Not short term.  But over years and years those tiny scrapes hold water and the paint starts peeling off.  If you have any plant life touching your house don't expect your paint to hold as long.  But this isn't a HUGE deal, the difference might be paint job every 15 years instead of 20.
Accidents.   Powerwashing, scraping it while making repairs, whatever.  Little scrapes turn into big things over 20 years.  (another reason for the second coat)
Gutters.  If your gutters don't properly work then the water will go down the side of your house.  Paint will not last as long.
Sun exposure.  Paint will fade in hard sunlight, even the best paint.  If one side of your house has extreme exposure then the paint will not last as long there.  


There are just a lot of factors.  I would say the biggest ones being how well it was originally painted combined with the plant life around the house.  It is a science on where to plant trees and other things to properly shade a house but not cause other issues.  Can your paint job last 25 years in Atlanta?  I don't see why it wouldn't unless you have some issues mentioned above.