It depends how you use and maintain it. As already told you that I have been using it since last 6 or 7 years (washed few times) and still looking good.!
You just touch the cotton strands and you will feel plenty of dust accumulated and full duster looks in dark colour. I used to wash it using Surf and never used any Rejuvenator.
Roughly, I used to wash once in a year, which is sufficient as I have covered parking in my house and also use a car cover while it is parked which saves from collecting unwanted dusts on the car.
You just touch the cotton strands and you will feel plenty of dust accumulated and full duster looks in dark colour. I used to wash it using Surf and never used any Rejuvenator.
Roughly, I used to wash once in a year, which is sufficient as I have covered parking in my house and also use a car cover while it is parked which saves from collecting unwanted dusts on the car.
Just ordinary Surf - yes, it will loosen the wax. (I was just checking it and found that I purchased the duster in February 2010, when I was having the Ford Fiesta. so I used the duster for 10 years, which is a long period for a product like this even if the wax is missing)