It could be awhile before a bigger eruption, according to volcano history. For example, in the gigantic Tambora eruption of 1815, it had a 'moderately large' eruption on April 5th, but didn't blow sky high until April 10th. And more recently, when Mount Pinatubo erupted on June 15th 1991, earthquakes and tremors began in March, steam eruptions began in April, dome building and movement along with smaller eruptions began in early June before the big bang.
Interesting...no specific seismic signature before the eruption - this is a steam and ash (phreatic) explosion - and it looks like successive steam and ash explosions occuring. This event is much longer than the last three.
So, in essence, if this keeps up for a couple months, we should worry? (in Portland, here)