How to build another city and how much time will it take?

You can create a new game on your device or if possible use a different device. If you use the same device you must make sure that the game is bound to an external account (create an IM30 account) so that you can log back into it. The in the settings menu under account you can 'start a new game'.

The time will depend on what you want the new account for. It will take a few days to get it going, and a couple of weeks to get it to the point that you can start using is as a farm. After that the progress can be slow if you want to build it up.