There are 2 ways to ensure if your Team Chest event is over or not. Firstly, you should check the minimum requirement progress bar to see if all the items have been achieved by your team or not.
If the progress bar is full, that team event is completed.
Or, you might also check the time available for the event right beside the progress bar. There will be a time countdown to let you know how much time you have to complete the event.
If the timer has stopped the countdown, you should understand that the Team Chest event is finished.