The answer to this question would be the same as for any other piece of clothing you wear for a prolonged period of time while executing some demanding tasks – as often as you can. Ideally, you should throw your yoga pants in the washing machine right after you take them off at the end of the day. That said, you have to evaluate how much time you spent wearing them and was there any sweat involved. If the answers to both of these questions are negative, then you can wear your yoga pants once again before washing them. If the answers are positive, however, then you better throw them in the washing machine as soon as possible.
Bacteria tend to spread at the speed of the light when you keep your sweaty yoga pants on your closet shelf. As a result, your previously fresh pair of pants will hold onto the sweat and the smell, and it will be hard to get rid of in the future. If you want your favorite pair of yoga pants to last you longer, make yourself a favor and throw them in a washing machine on a cold or warm setting (depending on its material and label). The next day you will have a clean and fresh pair of yoga pants with no traces of your yesterday’s out and about running.