How long does it take to develop a system

Most systems take years to develop and a lot of systems never make it to the point of being used. Companies have to develop how the system will work, how much it will cost, how it will be used, and what technology is needed to make it. After it's made it goes through extensive testing to make sure it works with the proper units and is accurate enough to be used in combat. Who develops them-----How are they tested?