When the magma chamber is full of magma, the sides of the volcano bulge. Most of the time, there are earthquakes right before a volcano erupts. There are also gaseous fumes, like sulfur that smells really bad. An eruption can last from a few seconds to a year, even centuries. Stromboli in Italy has erupted every few minutes over the last 2,000 years.
Some volcanoes erupt all the time.
Most volcanoes erupt for three to four weeks and then rest. An eruption will stop as soon as there is no more molten rock, when there is not enough gases in the magma, or when the chimney is plugged with cooled lava. Volcanoes erupt in different ways. Some are quiet. Others are very forceful and send tons of stream, ash and rocks into the air.