When you should change your S2 filters is different for everyone. It depends factors such as where you use your S2 Mask, your health, the health of those you interact with, the duration of use, and personal preference.
We like to use this example. If you travel on a plane, or on a subway, change the filter every journey. If you only wear the mask twice per week to go to the local shops, you will be fine for a week or two. It's all about exposure.
High exposure to people = frequent filter changes.
Low exposure = less frequent filter changes.