Government's typically last about 250 years and then either stagnate or are replaced. Here's an essay making that case: http://people.uncw.edu/kozloffm/glubb.pdf, but it is also pretty obvious that governments don't last forever just by thinking about when they start and end. Even China, which has a more-or-less continuous political history going back thousands of years, still had periods of chaos during which no central government really existed and then periods of reconsolidation and expansion.
Modern governments in the West seem to be entering another period of crisis so I'm very skeptical that any global government would succeed in their place.
Modern governments in the West seem to be entering another period of crisis so I'm very skeptical that any global government would succeed in their place.