Lifetime is one word which means the total time of a being’s life from the time it is born until it dies. It can also be used to mean a really long amount of time. Lifetime can be used as an adjective or noun.
Though not listed in the dictionary, when it is a noun the plural is lifetimes. One reason it is not recognized may be the fact that no living thing can have two lifetimes, and when speaking about two person’s individual lifetimes, it would be more clear to say lives. However, when using the term in the informal definition of a really long time, one could exaggerate the phrase even more using the plural (e.g., it would take two lifetimes to get that done).
It is used in the phrase of a lifetime. The phrase is preceded by an object or action (e.g., chance of a lifetime). The phrase suggests that the action or object is so unique or special that it only exists once in a very long time, or that the opportunity will not come again in that person’s life and should be realized at that moment.
As an adjective the phrase turns into once in a lifetime. When it is listed after the object it describes, it is four words. When it is used before the object, it is hyphenated.
Your editorial board’s decision to endorse Congressman Cory Gardner for the United States Senate ranks as one of the worst endorsement decisions, not only by the Denver Post but by any serious newspaper, in my lifetime. [Huffington Post]
An estimated 19 percent of women and 2 percent of men have been raped at some point in their lifetimes, typically by someone they knew, a new federal study says. [Washington Times]
It was the trip of a lifetime for Wayne and Theresa Baker, six weeks in Europe to celebrate their retirement. [Brisbane Times]
They will visit museums and military bases, with time for sightseeing and shopping, Burgess said. Scott described the trip as a once-in-a-lifetime opportunity. [The Southland Times]
“If something is going on we really want to be there for it and it was really just a great experience it was once in a lifetime,” Megan Smith. [News Channel 9]