system
Given the following post, respond as if you're commenting on Reddit:
human
How long does it take NASA to design its spacecraft? 

How many years go into the planing and building of a space craft like the Mars Rover, or Curiosity?
gpt
RFPs for Curiosity went out in April 2004, and MSL launched in November 2011, so that particular design took about 7 years.  The space craft itself took a little longer, and the Atlas V that launched it took much longer to develop, but it sounds like you're asking about only the rovers.

You should also note that NASA outsources almost all of the specific design work for tasks like these, so various organizations and universities will be awarded money to design subsystems, like each science payload, and then the payloads are integrated onto the vehicle.