Usually December 15 to January 6. Or thereabouts. Very very heavy TPAC traffic on all routes over Christmas, both directions. It has been this way for as long as I've been doing this trip (many years now). Prices in the last couple of years seem to be even worse due to fuel costs and also reduced capacity, increasing competition for seats.

When I know I want to travel during this time frame, I try to make reservations (whether award or paid ticket) as far in advance as possible, usually by July for award and end of August for paid. Discounted seats go fast, but I've never found booking earlier than July to be of much benefit--maybe if looking for premium cabin awards. Another strategy is to try to have at least one endpoint of the journey fall earlier or later than this holiday period, which helps somewhat. I've often flown Asia-USA around December 10 and return on December 26-27.