Many hotels will check if the DND sign is up for 24 hours or more, and I don't mind that. Once in London, while I was ill, I left the sign up for most of a day and the housekeeper just left fresh towels outside the door, but it was less than 24 hours.

I did mind when I had checked in at 4:00 a.m. and a housekeeper barged into the room at 9:00 a.m. to yell "Just checking!"