Though the red supergiant stage is the beginning of the end of a main-sequence star like our sun, and only after the better part of ten billion years have elapsed, the burns stage as a giant is still an awesomely long span of time from a human perspective.
But a 15% diameter change in less than two decades...that's quick as hell even for a massive star. By at least one estimate, Betelgeuse could go supernova at any point between tonight and ten million years from now.
Then again, quoting the sources cited on Wiki-
It is possible that Betelgeuse will become a supernova, which will be the brightest ever recorded, outshining the Moon in the night sky. Considering its size and age of 8.5 million years, old for its size class, it may explode within the next thousand years. Since its rotational axis is not toward the Earth, Betelgeuse's supernova would not cause a gamma ray burst in the direction of Earth large enough to damage its ecosystem, and also because of its 640 light year distance.
As has been often repeated, no supernova have gone off relatively close to us since the invention of the telescope, but obviously those numbers are going to catch up with us sooner or later. If Betelgeuse goes up, it's going to be a sight for the ages, in both the night and daytime sky.