My disclaimer on this is every airline is different and airfares change day to day and sometimes hour to hour.
I would start trending the airfare now - so you have a baseline in airfare. Then track once a day for a week. What I have found is booking on Mondays and Fridays typically means booking a higher fare. There is no magic number or time period, but I personally have found for my cities 10 weeks out is when I have found the most reasonable rates.
Like I said, this is my personal experience with air from the Chicago airports. I can be different for other cities.
I agree, if you find an airfare that is reasonable in your opinion, then jump on it. If it still seems high then wait. You are going in December so things may fill up quicker so keep an eye on it.