I've seen rust through on 2000+ Taurus' around the rear wheel wells.
And yea, it all depends on the weather conditions and how its cleaned. I picked up my Century from Florida this year and from there it has virtually no rust so I find myself washing it often, even every day, when the roads are salted and wet. I kneel and wash the undercarriage, floor pan, fuel door area, wheel wells, etc out with hot water.
I also make sure to rinse off under the hood like around the shock towers but most importantly underneath the lip of the doors, tailgate, and hood, where vehicles most commonly start showing rust at.
I find myself not driving my Forester every day but this winter I've found myself washing it off after every trip I make out...both vehicles should last pretty long at this rate.
For those that don't encounter road salt it's still good to wash out things underneath. Mud can get caked up and hold moisture. I'm undecided how sea salt has an effect.
Modern vehicles have really good primers, base coats, paints, clear coat, etc so I think it would be difficult to treat a vehicle badly enough to tap into the perforation warranty.
Some vehicles just do rust out in certain areas far sooner that the rest...take 1st and even 2nd gen Ford Explorers...first to go are the dog legs (area in front of the rear wheels)