Mining will never really be unprofitable, because the size of the pool will just regulate itself as needed. As the price falls, people running more efficient mining setups (read: cheaper electricity) will get a larger % of profits as others drop out.
At some point this has to stop as bitcoin ASIC manufacturing reaches top tier of available technology. Not sure if this moment already came so it's risky, but at some point energy and energy generation hardware will dominate the cost of mining bitcoin.
Here's a 10-Q filing from a miner from yesterday[1]
They mined 1,047 bitcoin from July, Aug, Sept. Their cost of revenue for mining works out to $13.5k per coin. But that doesn't include the cost of the equipment they use (which, by value, is mainly the ASIC miners). If you add in the figure that they provide for equipment depreciation, it's an additional $25k per coin.
Pretty hard to say since it depends on the price you pay for electricity. But you can just watch the difficulty adjusting to see when people have started to turn off miners
It is now; machines are being turned off as even the marginal(electricity) costs are now more than the return. Mining in case the price goes up doesn't make sense if you can just buy bitcoin for less than the electricity cost, of course.