It really depends. I have had one since November that is still in me. It is about 6 mm which is on the verge of "unpassable"...but it isnt bothering me and it is making progress. I go to the urologist once a month, and so long as it isnt causing problems he is willing to see if it passes. I am more inclined to try and let it pass rather than go invasive unless necessary.

Others can pass quite quickly, it really depends. A lot of it has to do with the size of the stone and its position in the kidney. Best of luck if you have one!!