If I or anyone else knew the answer to that question, we would be selling that advice. Remember you will be hitting peak season so don't expect much in deep discount like you might see in February or March. Old conventional wisdom was about 90 days out but not seen that working much these days. You have a lot of consolidation and reduction of capacity so there will not be that many seats available for last minute fire sale. Personally I would watch the fare for next four, five weeks and buy no later than end of February. The longer you wait the less flexibility as to route and seating you will find.