According to an OPEC site, the world has 4.5 x 10^12 (4.5 trillion) barrels of "potentially recoverable oil". See
http://www.radford.edu/~wkovarik/oil/ This is not the conventional "proven reserves"; this includes all known deposits of oil in any form that is potentially recoverable, e.g. oil sands.
1 barrel = 5.6 cubic feet.
http://www.eppo.go.th/ref/UNIT-OIL.html
So the world has 25.2 trillion cubic feet of potentially recoverable oil.
1 mile = 5280 feet. 1 cubic mile = 0.147 x 10^12 (0.147 trillion) cubic feet.
So the world has 25.2 / 0.147 = 171 cubic miles of potentially recoverable oil.
Not the 10,000 cubic miles that snowman says (but won't/can't provide any basis for).
Let's suppose the world currently uses 1 cubic mile of oil per year, as he also says.
At what rate will annual consumption grow? Look at three scenarios: +2%/yr, 0%/yr, and -1%/yr.
At +2%/yr growth in oil consumption, 171 cubic miles runs out in 75 years. At 0%/yr growth, it runs out in 171 years (duh). At -1%/yr, it runs out in - well, basically it never runs out, it asymptotes.
So in the +2%/yr scenario, if oil consumption grows at somewhat below current global GDP growth, the oil will run out in very roughly 75 years, even if we can recover oil sources that are today unrecoverable. At some point in that 75 years, oil price will soar, due to growing scarcity and rising cost of extraction. As oil breaks $100/bbl then $200/bbl, oil consumption will be forced to fall. Recession/depression, gas shortages, falling GDP, etc. So we won't ever actually "run out" of oil. It'll just be traumatic to get there.
In the -1%/yr scenario, if conservation and alternative energy combine to very gradually reduce oil consumption, the oil never threatens to run out. Oil price doesn't inexorably soar, though short-term things like MidEast instability or OPEC action still matter. Same as the first scenario, we never "run out", but getting there isn't traumatic - doesn't require $200/bbl oil, gas lines, oil-induced recessions/depressions, etc.
Anyway, that's my thinking.