Over 80% of people think they have a book in them (not literally, of course). An idea that will, one day, manifest into words, pages/bytes, a coherent and cohesive experience. Your house-sitter, manicurist, and that curmudgeon of a neighbor, wait patiently for retirement/the kids to go to college/the family dog to learn Mandarin before starting on the wondrous journey.
Those of us already writing know most of that 80% will never type a word. Some will begin and end with chapter one, realizing actually writing is different than the daydreams over coffee. Others will stop when they search for their muses, trying in vain to reach them while those traitorous wisps cavort on beaches with sexier writers than them.
Then, there are the few who find the linguistic fire late in life, buried under thoughts of propriety or of writing as frivolous.
It is good for the rest of us that the vast majority will never write a book (competition is already fierce) but it's slightly problematic, too. With so many people believing they'll just "jot down" a novel when they have "a free month or so", it lends false credibility to the illusion the writing life is easy or unimportant because almost everyone feels they can accomplish it. All it takes is some leisure time and a good idea, right?
So... tell the people who tell you they'll write "when they have time" to start today. You write when you have other things to do, all writers do. You could ignite their desire for words or, at least, help them see what it's really like after they set fire to the page (maybe, this time, literally).
Do people in your life say they'll write "someday"? What do you tell them?