I try (and I stress try) to clean my walls once a year and spot clean when I notice hand prints and dirty marks. I personally am a bucket and rag cleaning person but I've seen a few people use a flat mop with success.

My personal method: dust the walls first, bucket of water, minimal amount of either dilutable all purpose cleaner or, dish soap well mixed into the water. Use a rag to wipe down the wall, use another bucket of just water with a rag to rinse (unless your cleaner does not require a rinse).

If you have children or pets, you might need to wash the lower part of your walls more often.

I'm not shaming anyone who doesn't clean their walls annually. I know many people who don't. I just used to clean houses for a living so, I do this sort of thing annually.