Professional game developers require a few months to design a new game. But there are some exceptions. "Special" games like Spores require a year, at least.

The last two games I did took weeks and they were team projects for my software engineering course. I would rather take a few months to make a game though. If it takes you longer than that you either don't have enough time to really devote yourself to it or you are just lazy. Hmm... in that case maybe I'd rather take years. Of course, it all depends on your ambitions.

Blacktiger, the design process is different from the coding process. You design the game first, with details about how all the little things in the game tie together, then you code the game based on the design.

This topic has come and gone, so there's not really much to add to it. There's no strict limit on how long to spend on the design process - you stop designing it when it's done. The larger the game, the longer it takes.

How do you clone a game so that you can modify it? Lets say games for the xbox or xbox 360.

Play it, understand it, rewrite it.
What a question...

I tend to make my games simple. Very simple. Mainly they're all based on ONE idea, which i think is interesting and not too long to code. This is good because it makes you finish your games quite fast, it's bad because they might lack variety and its hard to add features without ruining the gameplay