How long do eruptions last?

Histogram of 3,211 eruption durations. Diagram reproduced with permission. From Simkin, T., and Siebert, L., 1994, Volcanoes of the World: Geoscience Press, Tucson, Arizona, 349 p.

Historic eruptions have lasted less than a day to thousands of years. In 1977, the lava lake at Nyiragongo drained in less than one hour. In contrast, Stromboli has had a low-level of activity since 450 BC (about 2,400 years).

Percent of Volcanoes    Duration 
        9%           <	 1 day 
	16%          <	 2 days 
	24%          < 	 1 week 
	30%          < 	 2 weeks 
 	43%          < 	 1 month 
	53%          < 	 2 months 
	17%          > 	 1 year 
	7%           > 	 3 years 
	0.5%         > 	 30 years 

The median duration of historic eruptions is 7 weeks.

Simkin and Siebert (1994) make several important observations:
1. the paroxysmal phase of an eruption can occur during any interval of a volcanoes eruption, for example the 1980 eruption of Mount St. Helens and the 1883 eruption of Krakatau were preceded by months of low-level activity;
2. some eruptions can reach their paroxysmal phase within an hour after the eruption starts, for example the 1886 eruption at Tarawera and the 1977 eruption of Usu;
3. to uninstrumented observers, unrest prior to some eruptions can be too short to provide a warning of an impeding eruption - highlighting the need for careful instrumental monitoring of active volcanoes.

Other volcanoes that have been erupting for the past 19 years (and maybe longer) that are likely to remain active in the future are:

Erta Ale, Ethiopia Manam, Papua New Guinea Langila, Papua New Guinea Bagana, Papua New Guinea Semeru, Indonesia Dukono, Indonesia Suwanose-jima, Japan Sakura-jima, Japan Santa Maria, Guatemala Pacaya, Guatemala Arenal, Costa Rica Sangay, Ecuador Erebus, Antarctica The current eruption of Kilauea has been continuous since 1983.