I created this chart using all qualifying laptimes since 1950. The colorful left side shows how far off P2, P3, P4, etc. typically were from pole that season, with the color bar dying off at 2%. The right side shows the distribution of all qualifying laptimes compared to pole at every race.
1% is a bit more than 1 second around Spa and less than 7 tenths in Austria.
Pre-2006 is straightforward: whether qualifying was single- or multi-session, every driver could compete in all of them, so quali gaps were plotted as-is.
Since 2006 and the elimination format, some extra consideration was needed for changing track conditions and/or the effect of race fuel loads. Therefore instead of using the raw laptimes of drivers eliminated in Q1/Q2, I applied a session improvement term on them, which I defined as the 25th percentile of laptime improvements between successive sessions. (The reasoning is that the fastest half of participants aren't fully pushing in Q1/Q2, therefore the eliminated drivers' improvements would be less than the average improvement between sessions. I verified this on drivers who just made the cut and the correction is pretty accurate.)
The chart speaks for itself and shows the uncompetitive beginnings, the '70s heydays, the sprawling field of the late '80s and early '90s, and of course standout seasons like the crazy 2012 or the viciously dominated '88-89.