It is very difficult to tell how massive a star is when it becomes a red giant, and we do not know the mass of Betelgeuse. Since life span of any stage is dependent on mass, we therefore do not know.

Star death is not something that happens rapidly except in the case of a supernova. As far as I know, we do not have good methods of predicting supernovae until we start seeing the neutrino flux as the star actually begins to explode. For less massive stars, it is not a process that takes place over a time scale where humans live long enough to see it.