Vehicle washing is a Condition Based exercise, not Time Based.
I have a neighbour who washes cars every Sunday. Twit. They're not even dirty sometimes.
My cars can go for weeks without a wash and sometimes need washed 2 or 3 times a week.
My trust steed survived a 15 day, 2,000+ miles trip without a wash. Saw some rain, but mostly dry. I wouldn't have washed it during my trip irrespective of the weather.