Down and feathers are a natural product and all things natural degrade over time.  However, a good down duvet that has been properly cared for should last you 10-20 years; a good pillow should last you 5 years. Pillows don't last as long because of the oils in human hair and that they are compressed during sleep, while a duvet is not. To increase the longevity of your products you should use protectors and covers that you wash regularly. Synthetic duvets and pillows do not last as long, as the manmade fibres will compress over time and, unlike down, they do not regain their loft through simply fluffing of the product. A synthetic duvet should be expected to provide reasonable comfort for 5 years while a synthetic pillow should last up to 2 years.