Down can last for decades. In a duvet particularly, you have the cover protecting it from body oils and dirt, so it very very rarely needs washing. I do like to air mine in the sun now and then when I am washing the cover if it is a nice enough day and flip it over a couple times to let the light hit everywhere. You can also wash down and dry it (you will need to put in something like a tennis ball to fluff the feathers and keep them from clumping). It comes out fluffy and good as new after you give it a good shake.

I don't actually find most synthetics to be long lasting. If you research sleeping bags, what we backpackers find is that synthetic fill bags lose their loft over time and no amount of washing can bring it back. Sometimes it is preferable if you anticipate a really wet environment, but that not really a concern for your bedroom usually.