What does it mean for a zodiac sign to be in retrograde?

In an individual's natal chart, if a zodiac sign is in retrograde, it means that the energy of that sign will be contracted due to this retrograde. In simple terms, a zodiac sign being in retrograde generally indicates that your luck during this period may not be very good.
However, the retrograde of a zodiac sign does not last forever. Your luck may be bad during this time, but it will start to improve after some time. Therefore, the luck of each zodiac sign is constantly changing. Even if your luck is not good recently, there is no need to worry too much, because the next period of luck may be better.

How long does a zodiac water retrograde usually last?

A zodiac water retrograde usually lasts about 20 days!
From an astronomical perspective, each planet has its own retrograde cycle, and Mercury's retrograde is no exception. From Earth's perspective, Mercury appears to go into retrograde once every three or four months, and each such retrograde cycle typically lasts about 20 days.
In 2016, there were four instances of Mercury's retrograde, with a total duration of approximately 80 days. The last one lasted until January 7, 2017. Mercury's retrograde is not actually the planet moving in the opposite direction, but rather an "optical illusion" caused by the difference in the orbital angle between Mercury and Earth's orbit around the Sun.