Instantly, very quickly. Lambs surely were known to be frisky creatures long before, but this expression, often shortened to in two shakes, dates only from the early nineteenth century and originated in America. Mark Twain changed it in The Adventures of Huckleberry Finn (1884) to “three shakes of a sheep’s tail,” suggesting it was already very well known by the late nineteenth century. A similar cliché, in a trice, which came from a now obsolete word meaning to pull on a rope and alluded to a single pull, is rarely heard today but was extremely common from the eighteenth century on.