Please explain in detail, or provide an an estimate, at least.
Currently the hash rate is around 25 TH/sec. It is expected that when the ASIC hit the market in Dec 2012, the new hash rate will be about 250 TH/sec. The difficulty is directly proportional to the hash rate, so it should also increase 10x. This would reduce the profitability of existing GPU miners by 10x. Plus about the same time the block reward will be halved from 50 BTC to 25 BTC, again halving the profitability.
The expectation is that once ASIC takes hold most of the GPU and FPGA mining will bleed out of the network. You should expect to see an initial influx of ASIC and then a steady growth as mininers reach for their larger share of the market.
Within months of the arrival of the first ASICs, we should see the mining difficulty rise so high that GPU mining is completely irrelevant. ASICs are up to 100s of times more power efficient, if you believe the numbers circulating around the forums.
Why months? It will take time for the first units to be shipped, installed, and to begin mining. But I know that several groups have invested heavily in up front purchases of ASIC miners, so the ramp ought to be steep.
At the current exchange rate, GPU mining can still be profitable if electricity is below twenty cents per kWh.
In less than three weeks, the daily mining revenues will drop by half in an instant (the block reward subsidy "halving" event, around November 29th). Difficulty may drop also as many GPU miners quit, but not much. So the breakeven for electricity will probably be about ten cents per kWh.
If you are a GPU miner and pay more than seven cents per kWh, continuing to mine will either be a waste of time (not profitable) or money-losing (if you pay above ten cents per kWh).
From there, ASICs will push the breakeven for GPUs down further ... to where the few people paying three cents per kWh can't even mine on GPUs profitably.
But since we don't know the future exchange rate and ASIC shipping dates and quantities, none of this is more than just being general educated guesses.
The halving event's magnitude (50 to 25) and time (around Nov 29th, 2012) is known. That will eliminate a bunch of miners. Then for those still standing watch for when you start seeing ASICs shipped in volume. Just know that is the point at which GPUs are then just a couple weeks, at most, from being obsoleted (unless you pay zero for electricity, but even then it is just a matter of time before you power them down for good).