You are probably very familiar with the noun die Zeit, usually translated as "time," but may not know that there is also a preposition with the same spelling: zeit. We covered this to some extent in a previous lesson, but it's time for an in-depth recap. There is some potential confusion with expressions that use the noun die Zeit with a genitive modifier that we should look at first:
Das wird die Zeit meines Lebens.
It will be the time of my life.
Caption 18, Glasperlenspiel - Geiles LebenPlay Caption
This very straightforward expression, "having the time of your life," meaning that you are enjoying yourself very much, is common in English too.
However, if you were to use the genitive preposition zeit in an expression that appears to be very similar, it would in fact mean something entirely different:
Und trotzdem hatten Morisot und ihre drei Mitstreiterinnen zeit ihres Lebens mit ihrer Malerei Erfolg.
And Morisot and her three fellow campaigners were nevertheless successful all their lives with their painting.
Captions 73-74, Malerei - ImpressionistinnenPlay Caption
As you can see, the preposition zeit means, in a general way, "during the whole course of," so the literal translation "during the whole course of their lives" is more simply translated as "all their lives." The genitive form thus renders this standard expression in the following forms:
— zeit meines Lebens ("all of my life")
— zeit deines Lebens ("all of your life")
— zeit Ihres Lebens ("all of your life")
— zeit seines Lebens ("all of his life," or for neuter subjects like animals, "all of its life")
— zeit ihres Lebens ("all of her life" or "all of their lives")
— zeit unseres Lebens ("all of our lives")
— zeit eures Lebens ("all of your lives")
Luckily, the preposition zeit is not commonly spoken, and nearly always in the above context when used at all. Remember that if the definite article die is used, then it's referring to having a good time, whereas the lack of definite article will be a good indication of its use referring to the span of a lifetime.
There's another expression for "all of your life" that may also appear a bit odd to the non-native German speaker:
Da sammelte es sich die Taler hinein und war reich für seinen Lebtag.
Then she gathered up the thalers and was rich for the rest of her life.
Caption 35, Märchenstunde - Die SterntalerPlay Caption
You can remember the meaning of Lebtag more easily if you think of it as "the rest of the days of your life." If you are curious as to why es is translated here as "she" and seinen as "her," you should read the Yabla German lesson "Possessive Gender Benders."
Go to Yabla German to find other examples of how the noun die Zeit and the preposition zeit are used in real-world contexts, and read the full definition of the preposition zeit on Duden.