Mercury's apparent retrograde motion lasts 2-3 weeks. Venus retrograde lasts one month or thereabouts. Mars retrograde lasts for about 3 months. These time differences are to be expected given the distances and positions with respect to the sun and earth. But look at all the other planets. Jupiter's, Saturn's, Uranus's, Neptune's and Pluto's all last around 5 months. I'm sure there's a mathematical explanation for why the apparent backwards orbit of the outer planets are the same length no matter how far away they are, but I'm not sure what it is.
Edit: Clearly I can't count. Jupiter's is four months. Saturn's is 4.5. Uranus's is 5.0. Neptune's is 5.0. Pluto's is 5.5. Still, the outer planets are puzzling, since they are at vastly different distances. Maybe their actual orbital speeds are different, but I'm not sure that accounts for it.
Here's the schedule:
Date Planetary station Exact degree
Dec 31, 2008 Saturn turns retrograde 21Vir46
Jan 11, 2009 Mercury turns retrograde 07Aqu45
Feb 1, 2009 Mercury turns direct 21Cap44
Mar 6, 2009 Venus turns retrograde 15Ari27
Apr 4, 2009 Pluto turns retrograde 03Cap18
Apr 17, 2009 Venus turns direct 29Pis11
May 7, 2009 Mercury turns retrograde 01Gem44
May 17, 2009 Saturn turns direct 14Vir54
May 29, 2009 Neptune turns retrograde 26Aqu28
May 30, 2009 Mercury turns direct 22Tau52
Jun 15, 2009 Jupiter turns retrograde 27Aqu01
Jul 1, 2009 Uranus turns retrograde 26Pis37
Sep 7, 2009 Mercury turns retrograde 06Lib13
Sep 11, 2009 Pluto turns direct 00Cap39
Sep 29, 2009 Mercury turns direct 21Vir36
Oct 13, 2009 Jupiter turns direct 17Aqu09
Nov 4, 2009 Neptune turns direct 23Aqu41
Dec 1, 2009 Uranus turns direct 22Pis42
Dec 20, 2009 Mars turns retrograde 19Leo41
Dec 26, 2009 Mercury turns retrograde 21Cap47
Jan 13, 2010 Saturn turns retrograde 04Lib39
Jan 15, 2010 Mercury turns direct 05Cap33
Mar 10, 2010 Mars turns direct 00Leo17
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