Current difficulty lasts for 1800 more blocks.
If it keeps increasing beyond 2 million, some rigs will never pay themselves off even if ran 24/7 unless value of bitcoin is increasing radically.
That will drop mining capacity until difficulty goes down again.
Approximently how long will these 1800 blocks last and how long do you think it takes for the difficulty to increase beyond 2 million?
11-12 days, give or take some hours. Current estimate is too low because at the end of block curve (~500 left), collective thash/s has always increased by nearly a third. 750000 is more realistic than the current estimate of 670000 (which is calculated at a constant 6.56 blocks per hour)
For 2m difficulty, ~70 to 90 days, but much more if $/BTC stagnates, because this will make 1-gpu miners drop out of pools due to very low profitability, which will make huge impact on difficulty (it will drop to 1.7m or less). The vast majority of miners are casual hobbyists with a few hundred mhash/s at maximum, not huge mining farms.
If many of them deem bitcoin to be too expensive to create in comparison to electricity costs it will push difficulty down and keep it in check until price matches again.
Also, if price crashes at massive difficulties, there is relatively low risk of big farms hogging the hashing power to themselves. They have no more guarantee than the hobbyist that the price will ever come up again so they would be mining at a loss creating something potentially worthless in the future. But that scenario has yet to happen.
