EDIT: I used the DE431 ephemeris to get the results below. However, if you visit HORIZONS (https://ssd.jpl.nasa.gov/horizons.cgi) and create an ephemeris for Pluto (actually Pluto's barycenter, object 999), the results include this warning:
The Horizons output log may report using DE431 as the source of target
or center data, but it is nevertheless using DE433 as an override if
Pluto is involved. This is to maintain ephemeris consistency for the
New Horizons spacecraft encounter at Pluto.
Therefore, my calculations may be inaccurate, above and beyond the normal inaccuracy of predicting planetary data in the past and future.
There are 2 times in the past 15,000 years and 5 times in the next 15,000 years where all 8 planets are in retrograde as observed from Earth.
The table below lists the times when this happens:
The last column is the ephemeris time, the number of seconds since '2000-JAN-01 12:00:00 UTC', which NASA refers to as "the epoch".
The first line in each pair shows the start time of when all 8 planets are in retrograde, and the final planet to enter retrograde to make this true.
The second line in each pair shows the end time when at least one planet is no longer in retrograde, and the planet whose retrograde period has ended.
B.C. 12435-AUG-10 07:52 JUPITER STARTS RETROGRADE -455482152466.038696
B.C. 12435-AUG-24 17:12 MERCURY ENDS RETROGRADE -455480909251.797852
B.C. 4139-MAR-23 15:02 MARS STARTS RETROGRADE -193692373029.630005
B.C. 4139-MAR-28 21:57 MERCURY ENDS RETROGRADE -193691916114.978455
A.D. 7662-APR-27 19:33 MARS STARTS RETROGRADE 178685595220.195740
A.D. 7662-MAY-12 05:19 NEPTUNE ENDS RETROGRADE 178686840014.478882
A.D. 8807-JAN-07 02:02 MERCURY STARTS RETROGRADE 214808680981.726929
A.D. 8807-JAN-20 01:05 JUPITER ENDS RETROGRADE 214809800771.851685
A.D. 10575-FEB-14 21:34 VENUS STARTS RETROGRADE 270604748086.593933
A.D. 10575-FEB-24 18:53 NEPTUNE ENDS RETROGRADE 270605602468.037781
A.D. 13679-NOV-06 16:03 VENUS STARTS RETROGRADE 368580427477.379639
A.D. 13679-NOV-19 01:05 MERCURY ENDS RETROGRADE 368581496795.683594
A.D. 16364-MAR-11 19:25 MERCURY STARTS RETROGRADE 453290109945.561157
A.D. 16364-MAR-13 08:53 MARS ENDS RETROGRADE 453290244824.895996
Notes and caveats: