It's not just the design life but usually the life of the project itself. When you hear that the Curiosity rover was designed to last 6 months that's the amount of time they essentially budgeted for for the project including wages for the teams that operate the hardware. Obviously by the time the rover arrives 18 months later the person doing the NASA budget can then ask for more money to expand the project timeframe, and as long as the hardware continues to function this generally happens YoY.