The frequency of cleaning the outside of the a/c depends upon who is in charge of the company at the time. Under some CEOs and the financial condition the country is in at the time, planes were kept cleaner.
Some of you may remember NWA during the '90s. We were trying out new paint jobs and paint quality. A couple of 757s were being tested and the paint was peeling off the test planes in a matter of weeks. I used to see them mostly in DTW.
We begged the company to pull those planes and get some new paint of them! While the planes were mechanically sound, passengers still questioned their airworthiness just by their look.
I used to shoot notes to the boss, if planes were looking dirty. I like to think I made a difference, sometimes.
NWA FA