@Diego33 and @Ed-and-Hugh0 Ooooh, what an great question! We have two duvet covers (with a third really sad one waiting in the wings in case of a laundry emergency) and we almost always change it in between every set of guests. My confession is right there in that previous sentence -- "almost always". Yes, I'm admitting that we sometimes (maybe 3% of the time) skip a change-out. We only do this if a) the upcoming guest is a single male, b) we're really pressed for time, and c) a thorough visual examination and close-to-the-nose sniff test all along the top edge reveals that the duvet cover is one that we, ourselves, wouldn't mind sleeping under. (To be clear, unlike some European hotels, we don't just use a duvet on top but also use a top sheet under the duvet.)
I don't *like* it that we sometimes do this. I'm trying to reduce this to zero. I fully believe that we should provide a fresh duvet cover every single time.
I have to add that I've stayed at places that didn't even *use* duvet covers. And you can bet they didn't launder that duvet in between guests...