For how long should one miswak be used? And does it become smaller with frequent use?

There is no specific limit to using the miswak. As long as it fulfills its purpose of purifying the mouth and removing bad odors, there is no harm in benefiting from it. However, if it does not remove the odor or food particles, or if its taste or smell changes, or if there is a fear of harm from it, then it should be changed by cutting it or replacing it entirely. The important thing is that it should be soft, cleanse the mouth, and not be harmful or disintegrate, even if the pungent substance is no longer present in it, because it is a purifier for the mouth and pleasing to the Lord.