Goto My Settings->My Challenge Settings.
You can specify a minimum timeout but not minimum timebank. I want to play 1 day timeout games and 7 day timebank games as I want to finish games fast but at the same time want to ensure some safety.

In short, I want to specify a minimum 7 day timebank in my settings. But this isn't possible now as much as I know.

Or maybe a option of a 3 day timebank should be allowed when starting a new game. Timebank starts with 7 days as the minimum. However, there is a 3 day timeout option available.