Over the last 500 years, it seems that eruptions are happening more frequently, but this could also be because more eruptions are being recorded.
When a volcano has not been active for a very long time, it is extinct, or dead, meaning it will not erupt again. A dormant volcano is only sleeping, and may explode once again at a future date. Sometimes people are surprised when a dormant or extinct volcano suddenly comes to life again. Mount St. Helens in Montana (United States) had been dormant for 123 years when it suddenly erupted in 1980. The explosion was as large as 500 small atomic bombs, but luckily there were very few deaths.
Mt. Vesuvius in Pompeii, Italy, has erupted at least 50 times in the last 2,000 years. Time and again, fire has rained down on the surrounding countryside. Today, more than 2 million people live in the beautiful region, near the active volcano. Following the 1631 eruption, the viceroy of Naples had the following message engraved on a tablet: “Sooner or later this mountain takes fire. But before this happens there are mutterings and roarings and earthquakes. Flee so long as you can. For soon the mountain will burst apart and bar the way for those who are slow to flee.
Forces of Nature: ThinkQuest 2000 (Team #C003603)
http://library.thinkquest.org/C003603/english/volcanoes/howoftentheyerupt.shtml
It is very difficult to tell exactly when a volcano will erupt. However, very large eruptions occur much less frequently than small ones, because it takes longer to build up the necessary pressure for a large explosion.