it never stops. that idea that one day it stops is not real, I think.
because in each existance you create new karma, or you still have some left from some other life. I don't think we always deal with every aspects of our karma in just one life, it could be very painful and emotionally distressful, more than what the soul is supposed to take at that moment.

the force is to keep reborning, non stop.
jeeez! you have no idea the different combinations there can be if you consider factors like family background, education, feeding, health, school, relationships, love, etc.