What do you consider “the big one”? We have earthquakes everyday and no way of telling how big anyone of them will be.
You can't predict earthquakes. It can happen now, or it can happen in another 100 years. No one knows.
California gets big quakes about every 20 years. They have all been in confined areas, devastating there, but only 20 miles away, just a mild shake. https://sslearthquake.usgs.gov/ens/
Between one nanosecond and a couple of hundred years. Shrug.
If people could reliably predict such things, we'd know.
Any minute now.