It stops when the company stops being profitable, or is worth more to a single investor than it is to the stockholders. If it stops being profitable, or even simply not profitable enough to justify the value of its assets, the assets will be sold off and the investors paid off (See: Knight Ridder). If it becomes more valuble to a different owner, they will make an offer, and the board will be forced to make the decision that maximizes the value for the shareholders (see peoplesoft, at&t).

Matthew