when does osmosis stop
It doesn't 

As long as liquid water is present osmosis will never stop, the closest you can get is a point at which it happens at equal rates in both directions, called equilibrium. 

Plant cells will help you understand as osmosis will create immense internal pressure as water enters the cell even though the cell cannot expand thanks to the cell wall. the amount of water inside the cell will keep rising until it reaches equilibrium, this also makes the cell more rigid.