Excavations into both terrestrial soil profiles and marine sediments have shown evidence of numerous past earthquakes on the Juan de Fuca subduction zone. Based on the various data sets it is evident that the average repeat time for earthquakes in this area is around 500 years. The maximum time is probably around 1000 years and the minimum time may be as little as 200 years.
It has been 300 years since the last major earthquake on this subduction zone. There could be another such quake tomorrow, or it may not come for several hundred more years. We do know that it will happen, but we simply don't know when.
From: Adams, J., 1990, Paleoseismicity of the Cascadia subduction zone: evidence from turbidites off the Oregon-Washington margin, Tectonics, V. 9, p. 569-583.