Time is measured in ticks.
In the game, there are 5 ticks per second (or one tick every 200ms), and there are 10 ticks to an in-game day.
This means each day is 2 seconds long in real time. So 100 days will be 200 seconds, or 3 minutes and 20 seconds.
Note: You may experience slight variations in the time depending on the performance of your machine, but on most modern machines and browsers this should be broadly accurate.