For Kairosoft games, after a specific amount of time (x years + start date [usually Y 1 M 4]), the game "ends" and your score is tallied. However, although your game has ended, you may continue to play.
Starting a new game, however, often has benefits. When starting a new game, specific traits from your old game are automatically carried over (often certain building/item/etc levels), which can not only make it easier to get a high score, but can also make it easier to play additional times, especially after having learned from the last play-through. Traits that are carried over from previous play-throughs stack, so potentially, it is possible to get 'perfect' play-through data.
Upon completing a game, you usually unlock the ability to speed up the game (Menu > System > Settings > Speed > Fast). In newer games, however, this is usually bought using micro currency.
For specifics on how a your score is calculated, what traits carry over between games, and when a game ends, this data is found on the Endgame page of the related game.
Should I restart my game?Edit
Ultimately, the decision is yours. However, here are some points to consider:
- When you have a game with IAP's you cannot get those tickets/medals/etc. back in your restart game