If you use your dish drying mat on a daily basis, it's important to wash it frequently. At a minimum, you should aim to wash your mat once a week. However, if it starts to look dirty or feel grimy before that, don't hesitate to give it a quick wash.
To clean your dish drying mat, simply toss it in the washing machine with some gentle detergent. You can also hand-wash it if you prefer. Just make sure to rinse away all of the soap before letting it air dry completely. Once dry, put it back in place and you're good to go!