by NFL rule, every flex game except week 17 must be decided 12 days prior to gameday. The decision must be made by Tuesday of this week. At this point, the NFL is notified and notifies teams so they may adjust travel plans accordingly. Then you will see announcements by NBC, the NFL, and the teams involved, so I'm sure we'll know by Tuesday night or Wednesday morning at the latest.
I'm still holding out hope for them to not flex this game. I want to be able to watch it live and won't be able to see the second half if it gets flexed, since my flight is scheduled to be in the air about halftime.