AG test isn't automatic. At least it wasn't last year. If it has changed then I am in fact wrong. In regards to how long you have, the email says until the Jan 5th deadline. However, it is strongly recommended to complete this as soon as you feel possible to do so.
I applied for the Direct TC last cycle and they held on my app (pre test), transferring it to this current cycle. So I can only advise you to complete it as early as you can. I plan to complete it before the end of this month as I don't want to spend too much time on prepping.
When did you apply in the last cycle? Late October?
Yesterday I received a friendly reminder email from AG saying to submit the application asap. Working on the answer right now to submit it until the end of this week.