Following is a summary list from the USGS quake database. As you can see there were 12 quakes of mag 6+ between October 1979 and October 1999, roughly one every 20 months. Although if we decluster for related events then there were only 9 events in those 20 years which is roughly one every 26-27 months.

It has been 123 months since the last one.

Brian

PDE 1979 10 15 231654.50P 32.633 -115.333
PDE 1979 10 16 065843.20P 33.016 -115.583
PDE 1980 06 09 032818.90 32.220 -114.985
PDE 1981 04 26 120928.40P 33.133 -115.650
PDE 1986 07 08 092044.50P 34.000 -116.610
PDE 1987 11 24 015414.51P 33.083 -115.775
PDE 1987 11 24 131556.40P 33.010 -115.840
PDE 1992 04 23 045023.22P 33.961 -116.318
PDE 1992 06 28 115734.12P 34.201 -116.436
PDE 1992 06 28 150530.70P 34.203 -116.827
PDE 1994 01 17 123055.39P 34.213 -118.537
PDE 1999 10 16 094644.13P 34.594 -116.271