According to this article it’s 12 days. I’m sure there are better sources… but a quick google search yielded this.I'm not positive, but I think there has to be a two-week notification before a game to be flexed, so this one will be played on Sunday night.
Explaining NFL's flex rules for Sunday Night Football
Denver Broncos fans were pleased to learn this week that the NFL has flexed the team’s Week 12 game against the Kansas City Chiefs to Sunday Night Football. The New England Patriots and New Y…
broncoswire.usatoday.com