Twice a year ... Around April, then September. They dry a bit quicker then. You can just hang them back up on the curtain pole, and they dry quite quickly.
It's bonkers to NEVER wash them. It's not about competitive cleanliness, who on earth never washes their curtains?! 
You ARE supposed to wash them!! This is the same people who never clean their oven, or their windows, and never clean behind the fridge. 😖
Why are some people on here so proud of being grubby, unhygenic, and slovenly?
First Google search brings up this...
Neglecting to wash curtains
causes a buildup of dust, allergens, pet dander, and odours, which degrades indoor air quality and can trigger allergies or asthma. Over time, trapped moisture leads to mould/mildew growth, while fabric becomes discoloured, dusty, and eventually deteriorates. Regular cleaning is necessary to maintain a healthy home.
Sort yourselves out! 😬
Life is NOT too short to do it. It takes no time at all hardly. Just stick them in the washing machine on a light mild wash. And the washing machine does all the work. All you've got to do is take them down, and hang them up. 15 minutes work tops - twice a year!!! And don't tell me they can't go in the washing machine. Nearly ALL curtains can! If you have velvet or silk curtains - which you don't unless you're the Queen - then get them dry cleaned!
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