Ok, we already experienced over the last 6yr's how bitcoin mining calculations went from mineable with a cpu, to where you need very expensive computer equipment to mine profitably. So what's going to happen to Bitcoin mining when virtually nobody can afford the equipment, and not to mention the calculations get so complex and difficult that it takes months or even yrs to complete a calculation? Only thing I can think of is Bitcoin exchange rate will increase to a point where 1 bitcoin would be worth millions, NO??
Mining is a self-adjusting system. The difficulty only rises in accordance to the available mining power. Hence, it can neither go to a difficulty where it will take months for a block to be found, nor can it become prohibitely expensive to mine.
Also see How is difficulty calculated?.
The cost of mining has actually decreased massively since CPU mining, which consumed something ridiculous like 5,000 Watts per GH/s. Today's best 16nm chips are speculated at running at less that 0.1 W per GH/s.
Second, the absolute cost of 1 TH/s of mining power has dropped significantly. You can pre-order an Antminer S7 batch 9 (~4.8 TH/s) for around 2.5 BTC.