Volcanoes don't operate on human timescales. They may go quiet for centuries, only to rumble to life with devastating eruptions. Their eruptions may last for days or decades, and it's often hard to predict in advance how long an event will last.
Officially, volcanologists consider a volcano active if it has erupted sometime during the Holocene Epoch, which started 11,700 years ago at the end of the last ice age. A volcano that hasn't erupted in the Holocene is considered extinct.
