The Earth's rotation is slowing down. In a few hundred years with the current system, we will need more than 1 leap second per 6 months. How many centuries is hard to tell, since it is affected by many things, including global warming.
But internal changes in the Earth can also cause changes in the rotation period. For instance the 2011 Tohoku earthquake shortened the day by 1.8 us [1].
[1] https://www.earthobservatory.sg/blog/how-did-2011-tohoku-ear...
Note that if the day slows by 5.5 ms from 24 hours, then we need an average of more than one leap second per 6 months to keep up. Per https://en.wikipedia.org/wiki/Earth%27s_rotation the Earth's rotation changed by 1.7 ms in the last century, an average of 2.3 ms in last thousand, and it is being affected by a variety of causes right now.