Answer:
Neptune has the longest duration of retrograde motion as viewed from Earth.
Mars has the shortest duration of retrograde motion as viewed from Earth.
Explanation:
The planets move along the space from west to east, although this movement is interrupted for brief intervals by a retrograde movement from east to west.
Retrogradation occupies a small part of the planet's motion that normally develops directly. The durations of retrogradation for the planets are: Mercury 23 days, Venus 42 days, Mars 73 days, Jupiter 123 days, Saturn 138 days, Uranus 151 days and Neptune 158 days.