Is there a limited time for using a siwak?

There is no set time limit for use of the siwak that one may refer to. Rather, so long as the siwak fulfils the purpose for which it is used, by cleaning the mouth of food particles and unpleasant smells, there is nothing wrong with continuing to use it, without any set time limit.

But if it no longer removes unpleasant smells or food particles that are stuck in the mouth, or if its taste or smell changes, or it has been used for such a long time that there is the fear that keeping it may cause some harm, then in that case you should change it, by cutting it, if possible, or replacing it entirely.

It says in ar-Rawd al-Murbi‘ ma‘a Zad al-Mustaqni‘: Using a flexible stick, whether it is fresh or dried, and whether it is taken from an arak (Salvadora persica) tree, an olive tree, an ‘arjun tree [36:39 was curved date stalk?) or any other tree, can clean the mouth and is not harmful, and does not crumble.

End quote from Hashiyat ar-Rawd al-Murbi‘ by Ibn al-Qasim, 1/148.

Shaykh ‘Abd al-‘Aziz ibn ‘Abdillah ibn Al ash-Shaykh (may Allah preserve him) was asked: Is there a set time for using the siwak, meaning that when it loses its tangy taste you should stop using it? Is it permissible for the Muslim to use it even when that taste is no longer there, and will he get the same benefits and rewards of using it?

He replied: We have not come across anything to suggest that. What appears to be the case is that it is prescribed to use it so long as the stick can still be used as a siwak and it purifies the mouth. “The siwak is purifying to the mouth and pleasing to the Lord.” [an-Nasa’i; Ahmad, 6/238; ad-Darimi, 684).]

And Allah knows best.