How often do volcanoes erupt?
Well that depends on the volcano. Some volcanoes erupt very often (and some like Kilauea almost never stop). On the other hand, some volcanoes are inactive for very long periods of time between eruptions. For example Mt. St. Helens erupted in the late 1800's and then again in 1980. That is considered a relatively short rest for volcanoes in the Cascade range. Pinatubo, however, last erupted about 400 years ago prior to its 1991 eruption. Lots of people didn't even consider it to be capable of erupting, it had become so eroded during those 400 years.
The largest eruptions come from volcanoes called rhyolite calderas, and these huge eruptions (which we haven't really witnessed since 186 AD in New Zealand) may occur at intervals of 10,000 to 30,000 years. Yellowstone, the largest caldera in the U.S.A. seems to erupt on average every 600,000 years!
Aerial view of Mount Pinatubo after the cataclysmic June 15, 1991. USGS photo by E.W. Wolfe.