Life Hack: How to wash a duvet and how often you should do it

Duvets generally last up to five years and if you wash it regularly and use a protector, you can extend its life.
When was the last time you washed your duvet? Was it when you spilled some coffee on it, or when toilet training a pet or a toddler went a little wrong?
Or with nothing like these examples happening, have you never washed it, assuming freshly washed bedsheets are enough?