I had my 5D4 for two years, never had to clean the sensor once.
Had the R two months and after taking a slew off photos a couple weeks ago, noticed a “smooge” in some of the frames. Thought it was a specific lens at first, then noticed same pattern across shots taken with different lens. Don’t know how it got there, was a pain to get clean (took about 5 tries)
I am pretty anal about only changing lens with power off, keeping body pointed down, etc. Don’t for the life of me know how it got goobered up but it was. Not dust, but a smear of something. Only really showed up in light areas (sky) if I boosted exposure or contrast.
I am lucky I clean my own sensors and have done for the past 10 years or it would have cost a fortune