as far as i know they mainly send test invites via email. Most of us (December applicants) are still waiting for Test invites, you can check previous months progress on the sheet to get an idea.
According to the reported data, 5% of December 2017 applicants got test invites, 25% of November 2017 and 58% of October 2017
So it might be very soon, or it could take several more weeks / months (hopefully not).
Thanks