Most certainly not december, December and January are the months you will probably see the biggest difficulty explosion ever, assuming at least some of the promised 28nm products make it to the market roughly on time.

When is it going to taper off? When marginal production cost meets mining profitability. Until then, ASIC manufacturers will just drop prices to maintain sales (causing difficulty to increase further, and requiring further price drops).
Cointerra is now on preorder for ~$2800 per chip. Im guessing variable production cost of the chip alone is closer to $10 per chip. Do the math were difficulty would need to be for 500GH at ~$10 (+PCB+ case+margin, say $25-$30 ) to be only marginally profitable. That gives you an idea. Then double it when 20nm becomes affordable.  The short answer is: not anytime soon.

edit; pallpark figure, when difficulty reaches ~100,000,000,000 is where 500GH for $30 is only marginally profitable, depending on electricity cost, BTC rate etc,  Take that figure with a grain of salt, but it gives you an idea. It may also taper off sooner, but OTOH I also expect we will overshoot this value due to inertia, free electricity and miner myopia.  Also, this is a race, until we get close to this point, the only brake on network growth will be the ability of the various vendors to produce and ship. Unless they are all as inept as BFL and avalon in getting their goods out of the door, that may go incredibly fast. If you thought 2013 was crazy, you havent seen anything yet.