Perspiration and bodily oils are the main causes of bacterial growth in your mat, resulting in unpleasant smells.
So, the regular rule of thumb is that the more you practice or the more you perspire during your practice, the more regularly you should clean your mat.
Here are more details:
- If you practice regularly: clean your mat once a week
- If you practice occasionally: clean your mat before each practice
- If you perspire a lot during your practice: clean your mat immediately after your practice