I liked how Sid Meier put it in an interview: > Our philosophy is to find the fun first and do a lot of prototyping and a lot of iterating very early on. There

I liked how Sid Meier put it in an interview: > Our philosophy is to find the fun first and do a lot of prototyping and a lot of iterating very early on. There are probably more projects that I've worked on that haven't turned into final games than have. > We have ideas. We have prototypes. We spend a lot of time trying out ideas. Probably two-thirds of them don’t end up going anywhere, but experimenting with those kinds of things is fun. To answer the OP question, I can't find the interview I was looking for now, but I remember that he said that he stops working on a prototype when he realizes that he's learned enough from it.