What are the smallest and largest predictions for when fossil fuels will run out?

[Chloe]

What are the smallest and largest amount of years that scientists have predicted fossil fuels will run out on?

[hcheese8]

not sure of the numbers your asking for but i know that in Canada there's enough fuel to last the whole world about a century

[Mountainboy19682]

The world will never actually run out of fossil fuels. However at some point they will become too expensive to use and will be replaced by some other fuel.
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Conventional oil will be replaced first. In the 1950s M King Hubbard developed his Peak Oil theory and famously predicted that US oil production would peak in 1970 - which it did. His prediction for world peak production was 2010 (see chart in link 1 - which shows conventional oil production running well past 2200 - but much at lower levels than now).
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In free markets, the price of a commodity is the cost of the highest cost producer supplying the last increment needed to meet demand. Right now the cost of oil is driven by the costs of bringing on new unconventional oil projects - primarily Canadian oil sands.
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In 2010, oil consumption was 30 billion barrels (82 billion barrels/day). Canadian oil sand reserves are only 182 billion barrels - enough to add 6 years to world oil needs. (See link 2 which is a report of all energy reserves).Reserves are always difficult to assess and there are probably much larger reserves in Canada, but certainly not the 100 years that Cheese8 is talking about.
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After oil sands, the next biggest unconventional resource of liquid fuels is oil shale. Reserves are estimated at 3 trillion barrels - enough for 100 years of current world consumption. But oil shale is expensive to mine and process. The likely price needed to justify projects will be well over $200 per barrel.
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Coal is another fossil fuel - and the most important fuel for electricity generation. World consumption in 2010 was 7 billion tons. Inferred resources are about 700 billion tons. Again there are probably far more reserves than 700 billion - but what is the cost of production?
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So the real question for optimists (which includes me) is at what stage will renewable energy (solar cells for power generation and biofuels for transportation) become economic. Many environmentalists claim we have reached that point now. A more sober assessment is about 50 years before fossil fuels are phased out because they are too expensive.
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The pessimists would ask at what point do the social and environmental issues of rising energy costs cause the collapse of our economic system and a return to the stone age. Possibly 50 years?