In the end, shale oil is also a finite resource. Fracking buys time, which is a good thing to have, but we will still have to transition to a new fuel source sooner or later. As far as I know, the long term still looks like a race between electric vehicles (with the unsettled question of how to generate the electricity to begin with), fuel cells (pending cost affordable solutions to a couple of problems), and biofuels (if prices come down on converting third generation feedstocks).
“(What is the) estimate for the expected life cycle of the fracking boom. The resource is big, but how big?”
The USA has a few trillion barrels of shale oil deposits, and we have a track record of finding more than we pump (total reserves have grown over time). That is more than the rest of the world’s total proven reserves of conventional oil. So just our shale oil is about 50 years worth of current US demand for oil. We also have additional regular oil reserves, world class natural gas reserves, and hundreds of years worth of coal.
The main factor in the frakking boom is price. If the price is high enough to sell it profitably ($45-$65 per barrel), producers will bring it to market (assuming that the government does not restrain them). When price drops too low (like earlier this year) they will start pulling in their horns. Due to the nature of frakking, they can ramp up production relatively quickly, but the longer prices are low, the slower the rebound (skilled people move to new jobs, equipment gets sold, etc.).
Oil shale varies widely in how easy (cheap) it is to extract - it is generally more expensive to produce than conventional oil, but technology has gotten better quickly. “Proven reserve” is a very high standard - sign a contract for timed delivery. A lot of our shale oil deposits would be harder to pump, and would cost more per barrel - but there is a LOT more of it, and we are finding lots more when we look.
We are not the only ones with Shale oil and gas, but we (and the Canadians) are the big active shale producers. As the technology spreads, others will start to produce more shale oil and gas (when prices favor the investment). Russians have bought technology and have huge deposits. Israel has significant deposits (not the best quality, but they also have big offshore conventional gas fields approaching production). China and Argentina, as well as most conventional major oil producers also have large shale deposits.
So the frakking/horizontal drilling revolution has changed the oil market fundamentally. Oil is much less scarce, and OPEC has much less control over total production or prices (above the shale price band).
The bottom line is that scarcity will not drive us off conventional hydrocarbon fuels like oil, gas or coal for a few generations - if ever. Cheaper alternatives are quite likely in that timeframe though - some analysts argue that solar cell efficiency is already reaching parity with market costs for grid electricty. Political actions (corrupt sellouts, global warming concerns, environmental restrictions on frakking) could drive us off hydrocarbons at high cost. But there is no realistic hard constraint on energy availability on the horizon.
Burn the coal, capture the CO2, turn it to fuel. Probably far more efficient than any of the so called "green energy" alternatives because there's a century of experience with using coal byproducts just sitting on the shelf (many of which are far more efficient now than in the past) because it was easier to transition to oil than to use coal.