How often are supernovas visible by the naked eye from Earth?
One way to answer this is to look at how often naked-eye supernovae were seen in the past. In the last 2000 years, there have been (at least) eight such supernovae recorded: in 185, 393, 1006, 1054, 1181, 1572, 1604, and 1987. Seven of these were in our galaxy, while the last (SN 1987A) was in the Large Magellanic Cloud.
It's possible that supernovae in the far southern sky would not have been visible to Chinese astronomers (the most reliable and comprehensive records we have), so some additional supernovae visible from the far south may have been missed. We could round the total up to ten, which gives us a rate of about one every 200 years. If you want to worry about some being visible but not recorded (e.g., because they happened during a particularly chaotic time in China), then perhaps the rate would be a little higher -- maybe one every 150 years?