From the wikipedia entry on DARPA: Although individual projects typically last three to five years, major technological challenges may be addressed over longer

From the wikipedia entry on DARPA: Although individual projects typically last three to five years, major technological challenges may be addressed over longer time periods, ensuring patient investment on a series of focused steps and keeping teams together for ongoing collaboration. Continued funding for DARPA projects is based on passing specific milestones, sometimes called “go/no-go’s. From the DARPA website: To maintain an entrepreneurial atmosphere and the flow of new ideas, DARPA hires Program Managers for only four to six years because the best way to foster new ideas is to bring in new people with fresh outlooks. New people also ensure that DARPA has very few institutional interests beyond innovation, because new Program Managers are willing to redirect the work of their predecessors – and even undo it, if necessary.