Parent seems to be assuming, rather, that earthquakes take time to reach "full force". I weakly suspect that's not the case, but it's a different question than "before it happens".
Even with the early warning, sometimes the earthquake will still be on the weaker end of the spectrum, so you brace yourself but not necessarily panic.
The 2011 earthquake started off moderately, so no one in my office ducked for cover until it really started to shake, which was already about half a minute in.