Sometimes when you do a long (>16 hour) travel there is a small chance that you will make it in exactly 16 hours. This uses up all your hours and the travel is instant, just like normal.
If this does not happen, then you instantly use all 16 hours, move 16 hours closer to your destination, and then get one hour closer to your destination every time you would normally have gained an hour in your time pool. When you are one hour away from your destination you will stop using hours, start gaining hours, but stay one hour away. At the next turn change you will arrive with whatever hours in your time pool that you had accumulated.