I am not a native english speaker so maybe I didnt express myself well
I believe that most of the people that are mining (like 70 to 90% of them) are people with average or mid grade salary (in real life jobs) and they have average to low grade hardware (most of them usb sticks,5 GH/s machines and some of them 500GH/s machines tops...)
So for those people if the difficulty exceeds lets say 6 or 8 billion they sure will quit since their hardware wouldnt even get them a thusandth fraction of a bitcoin per day in that difficulty so if and when they quit the difficulty should drop down (since 70% or 90% of the miners dont mine anymore having as a result the average time for a block to be found to increase noticably)
So at that time it should be a good idea do get into mining with a strong piece of hardware thats what I was talking about mine for a few weeks and then when people keep on coming back you just wait for the difficulty to raise up again.
You don't get it, the difficulty will not stop increasing whatever
people with average or mid grade salary do.
First, most of small miners don't stop mining even if their income is negative (higher electricity bill than income from bitcoins). They mine until death.
Then, even if the small miners stop because of high difficulty, it is too late to make any difference on difficulty because the total of their cumulated hashrate is totally insignificant.
Everyone tell you the truth : Mining is a very high risk investment. Most people loose money. Only a great knowledge of the bitcoin world can give you a chance to make a profit.