Up in MA. We tend not to wash much in the winter as it gets covered in salt again the next time we drive it. Have run numerous cars including previous LR4 and Discovery to 100k plus without a hint of rust anywhere. It all washes off great in the spring and paintwork looks immaculate
Think paint technology is vastly more resilient than in the past and alu panels definitely helps too.

 

Agree with BritCars, I'm in New Brunswick, just north of Maine and after a wash the Defender looks the same as it did prior to within a day or less (see pic below as to what it looks like most of the winter). I try and wash at least once or twice a month on a sunny day when its above 5 C (41 F). ​​​​​​​

 

I have a buddy who doesn't wash his pickup truck, it is now so badly rotted underneath that the shop told him it's risky to even tow with it this year which is the only reason he buys used pickups. Down south people are driving 15-20 year old trucks that look like new. It's a whole different world. I think you can get away with not washing much even in certain nothern areas of the US, even areas of Pennsylvania, Boston, Virgina seem to have cars in good shape and others like where I am in Western NY you'd be foolish to think it will not rot badly underneath. I have lived here all my life and the first cars I owned were so badly rotted underfoot that you could see the road through any small holes that were in the carpet. I'm looking at used carmax cars over the past few weeks and see plenty of cars in new england area which look very good. I also had one actually shipped last week from Virgina that arrived in really immaculate shape. Zero rust underneath or anywhere else for that matter. On the other hand I had a car 10 miles from me which looked to be in perfect condition in the pictures, paint looked great etc, relatively low miles, got there and looked underneath it.... wow.. Had to tell the dealership, umm about that test drive that I wanted to take..nevermind. Awkward moment.

Edit> Also if you aren't even going to keep it for more than 5 or 6 years anyway then I suppose it's the next owners problem. Even aside from the rust-prevention piece of it I just like my car to be clean because I feel as if it looks a lot better, if I paid 2-3k extra for Carpathian paint on the Defender but it always looks filthy what's the point? Is it worth a $30 per month wash subscription? In my area I would say yes it is.