You actually hit on a VERY big question there, and if one applies a little thought to it, there IS an answer: Consider the basics: the films A New Hope and The Empire Strikes Back established that the Force was an energy field created by all living things, that life both created it and made it grow. The book Star Wars: The Annotated Screenplays (p. 181) confirms this and specifies that the very act of living is what generates the Force. The simple point, made by Kenobi and Yoda, that life is what creates and sustains the Force can give us a rough idea – albeit on an unimaginably vast timescale – of how long the Force has existed and how long it is likely to exist, especially if one takes seriously the proposition that the Star Wars galaxy exists in the same universe as our own, though it is situated very far from our galaxy in both distance and time. The existence of the Force, it would seem, depends on the existence of life itself, so the Force would have to have sprung into elementary existence not long after the most elementary forms of life began to appear in the universe. The Force would have become larger and more powerful as life became more complex, and especially so when sentience first emerged. But if the Force was both created and sustained by life – indeed, by the very act of living – then the Force can only exist as long as life itself exists, and modern cosmologists are beginning to suspect that there is in fact a point when all life in the universe will cease. As life can exist in some of the harshest conditions imaginable, this will take an unimaginably long time to happen, but according to the Big Freeze theory advocated by most cosmologists, the universe is expected to eventually expand to the point where every element within it essentially runs down, achieving a state of complete equilibrium where there is no thermodynamic energy to sustain motion or life. At this point, called the heat-death of the universe, life – even the raw materials for life – cannot exist anywhere. The Force would thus begin to weaken and eventually dissipate as it loses its source of generation. It thus may be the last remnant of life to exist in the universe, but it too may fall prey to entropy. Of course, if Star Wars physics chooses not to acknowledge the heat-death concept, all that goes out the window; but more than one EU source has made specific mention of heat-death, so there is at least a precedent (even when factoring in how shaky the ground on which the EU stands is right now).