The timebank value is per game. It does not reset during the game.
The time per move however resets to its maximum value on every move you make.
So on a 3 days/move, 7 day timebank tourny, you can take anything up to 3 days for each and every one of your moves and that will be fine. If you go over the 3 day limit, then your timebank comes into play: your timebank will start to decrease once you excede the days/move limit. It stops decreasing once you move obviously! The timebank never increases during a game. Once your timebank reaches zero: it's timeout time and you loose.