The term 'retrograde' denotes the apparent backward motion of a planet. During the period when the transiting planet leaves direct motion and turn retrograde, the influence of transit is a little different:

For example, when transiting Uranus, Neptune or Pluto form an exact aspect to a natal planet generally we record the need for changes related to the facet of life associated with the natal planet. However, when the transiting planet leaves direct movement to turn retrograde, our efforts to make changes or adapt can be obstructed or blocked, and also our desire or need for change may decrease during that time.

The energy of the planet at that time, seems to induce more a contemplative type experiences of reflective nature, more introspective behavior or reserved than usual, and there may be a sense of "déjà vu" in terms of the nature of the planet, because a planet can pass on some degree up to three times.

The retrograde periods provide an opportunity to really get to the essence of a subject, because they force you to go to the core (bone) again and again. This can be frustrating and tedious, but at end is very useful to deal with some learning that we can be avoiding and which necessarily has to be addressed.

Mercury has the shortest retrograde phase, 22 days on average, three times a year.
Venus retrograde cycle is too short, 42 days and repeated every year and half approximately.
On Mars this cycle lasts about 2 months and is repeated every two years or so.
Jupiter remains around 3 months retrograde each year.
Saturn remains retrograde about 4 to 5 months each year.
Uranus, Neptune and Pluto remain retrograde around 5 or 6 months each year.