Just that, really. I’ve been getting the beds ready for winter and noticed our duvets are a bit lumpy. I also just purchased a new one for DD who’s now transitioned to a proper bed and realised how lovely a fresh duvet is!
Google tells me they should be replaced anywhere from every 2 (mad!) to 5 years. DH grew up with a mum who never replaced hers (in fact if we stay at hers, she’s still got the same duvets he slept with 20+ years ago, they’re like sleeping under rocks!)
I have asthma so tend to buy synthetic. I wash and air each duvet at the end of the season, if that matters.