2002 Sienna, almost 175,000 miles. For nearly 5 years, mostly parked in hot sun at 5,740 feet above sea level. Hasn't been waxed yet. This is a quarry town, and there is usually a layer of dust on it, so it would be hard for bird droppings to hit the paint.
I did wash it once this year. Looked pretty good. My usual policy is, when God wants my car washed, He sends the rain. Heh, heh.
It is pretty much a waste of time to wash it, unless there is some special occasion with important people to be transported. I wash it; by tomorrow morning it is covered with a layer of dust again.
The older cars so common here in Mexico, the GM models at least, as someone said, the paint is pretty much gone. The neighbors warned me my paint would fall off if I park it outside, I told them, It's a Toyota, not a GM.
My partly tongue-in-cheek theory is the dust I leave on it protects the paint.