Teachers hear this every year from their students. Is it a fair question to ask? Well, it depends on how the teacher is using their influence. Is the teacher saying that knowing the electron configuration for tungsten is important regardless of what your future holds? Or is the teacher saying that the specifics of the course may not be useful, but the methods that were implemented in arriving at "learning" the concepts will be used long into adulthood? That is a fair statement, unless the teacher is only interested in compliance and little else.
In one instance, the specific subject matter is the focus (learning how to complete the square in algebra, for example), and in the other, life skills are the focus. In the life skills model, the teacher uses the subject matter to teach inquiry (asking good questions), problem solving, collaboration (working together to a shared end result), grit* (sticking with a problem until it is completely and adequately solved), creativity, and communication- maybe one of the most important, but least practiced. Effective communication includes listening. Listening for understanding is losing popularity in this world of self promotion. If those skills are what is being taught, then the answer to the original question is, "For the rest of your life."

*Grit may be the most important of these skills. See more on grit here.