My expected timeline:

ASICs start entering in droves, hashrate and difficulty shoot up.
Remnant GPU miners switch to SCRYPT based coins (litecoin, worldcoin etc.)
BTC hashrate/difficulty drops a bit, and then goes back to it's current trend.

How soon it will happen all depends on how many/how soon the ASIC producers will actually ship any decent amount of the machines. Given that most of them were supposed to be out by now and very few have actually been seen (yet) in the wild, I would be careful about making any prediction about how they will affect the market.