Does a card not have time yet or do you want to change the time? You can easily add an activity to the card and set the duration in minutes (m), hours (h) or days (d). Select the activity icon in the quick menu.
An additional item appears at the bottom of the card. Select an activity and determine its duration. Click the purple confirm button behind the line to save the activity or press the ENTER button on your keyboard.
You can also directly change the duration of an existing activity. Go to the line and adjust the duration. This will be implemented immediately.
You have no activities? Then create them in the settings. Read here how to do this.