In short, yes. Like any item of clothing, with wear they will become dirty, pick up odours and general grime. This does not mean you should wash your outer garment every time you wear them, but you should take the time to give them a wash as needed. It is hard to give a time frame, as it will really depend how much you use them and how hard you are working. Some key things to look out for are the outer fabric wetting out (holding on to the moisture, not beading and letting it roll off). It is best to wash in a pure soap product, not a detergent, to protect the DWR (Durable Water Repellent) finish. Your local outdoor shop will be able to suggest some products that are suitable. You can also refresh this finish with a number of products should it not be performing well.