How long has Mt St Helens been erupting?

The eruptive history of Mount St. Helens began about 40,000 years ago with dacitic volcanism, which continued intermittently until about 2,500 years ago.

When did Mt St Helens erupt sideways?

May 18, 1980 Helens Eruption Was the Volcanic Warning We Needed. Its been 40 years since the sideways explosion that changed volcanology forever. On the morning of May 18, 1980, a volcano erupted not from its peak but from its side.