There has been many different ideas of different time limits on games, and what I believe is one of the best is what DailyGammon.com uses.

Quick overview of how they work:

Each games has a grace period. (Example - 24 hours) So for each move, you have 24 hours before any things starts to happen with the timer.

Each game also has a Total Time (Example: 100 hours). After the grace is over, the total time starts to go down. Once the total time is at 0, you time out and lose.

Some games also have a +TOTAL time. (Example +2 hours). Any time you play your turn while still in the grace time, you get +2 hours added to total time.

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There are some other minor details, but that is the basic of that sort of time limit.