Roughly once or twice per century
In a galaxy like the Milky Way, astronomers estimate a supernova goes off every few decades on average, though dust hides most of them. The last one bright enough to see with the naked eye in our own galaxy was in 1604, observed by Johannes Kepler. SN 1987A in 1987 was the most recent naked-eye supernova visible from Earth, but it exploded in a neighboring galaxy.
