"is it dirty?" is what I use as a good way to decide if my cars need a wash. I'm sure alot of people don't actually have a strict regime for when they wash their cars (I used to detail them every sunday - but that was just silly).
Since it is black, you will be washing it all the time (but it sure looks nice when you do)
I follow the "wash it when it is dirty" rule, except for situations where something like bird sh*t gets on it, which is a substance that is known to EAT through paint. Any other kind of protein-based contaminant should be removed pretty quickly also because these things have a tendency to chemically bond with paints/clear coats etc. Just ask anyone who's car was ever "egg'ed" by punk kids.
I also sprung for the extra $$$ and had the dealership put on a special paint protection clear coat on top of the factory clearcoat (mine is a product called "Guardian"). Besides making the car ALWAYS look like it has been freshly waxed, it came with a really solid guarantee that will get any wear and tear/fading to the paint job fixed at no cost to me should the issue come up later. If you are really concerned about the longevity of your exterior paint, you should look into something similar.
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