Reading watsisname's source, the X-rays from a type Ia supernova aren't dangerous until it gets relatively close to us; it's the gamma rays that are more of a cause for concern. Even so, I don't feel it's a huge cause. 50-100 pc might give us more reason to worry.
In any case, regarding Betelgeuse, it will explode sometime within the next 100,000 to million years. That's all we can say, because once a star begins fusing helium, and those changes have made their way to the surface, any further changes in the core will generally not be seen by outside observers because it takes too long for radiative transfer to bring us this information (i.e., the changes make themselves felt outside the core, but because they are dependent on photons to relay this news, and photons take a really long time to make it out of a star, we may not see it before the star explodes). For a 20 solar mass star like Betelgeuse, once carbon and oxygen burning commence in the core, it takes roughly 3-500 years for that supply to be exhausted and for the star to then begin fusing the products of that reaction (silicon, mostly) into heavier elements via alpha capture. That process lasts about 6 months, until a core of iron builds up to the Chandrasekhar mass. Once that happens, the core collapses in less than a second to become a neutron star, and the star explodes.
By contrast, the helium burning stage takes about a million years, and hydrogen burning about 10 million, again for a star with a mass like Betelgeuse's.
