The reserves/production ratio or R/P is a method used to assess the size of reserves. The value represents the number of years that current reserves would last if their rate of use did not change. The value of this ratio changes as the size of the reserve changes. A reserve is defined as the amount of a resource that can be extracted with current technology, at current prices. If technology improves, or prices increase, the reserves increase.
These R/P ratios are most important when analyzing fossil fuels, as it loosely represents how much longer they will last. Current estimates put the reserves/production ratio for coal at 119 years if consumption continues at its current rate, while there is 63 years of natural gas left and only 46 years of oil. The idea that oil is running out is linked very closely to Hubbert's peak, an estimate about when oil production would peak and how it would fall after a certain point in time.
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Although this number can sometimes provide an estimate of how much of a certain resource is left, the numbers can be inaccurate. First, this value doesn't account for any resources that are yet to be discovered. If found, these resources could dramatically increase the amount of a fuel in known reserves. This would extend the period of time over which a resource is expected to run out. Second, the number doesn't account for potential economic or technological factors that could expand the reserve. The cost of fossil fuels could rise or technologies could advance to the point that certain previously unrecoverable resources could be mined. This would change the value of the ratio. Finally, there is the fact that fossil fuel consumption rates are growing, so estimates today likely represent time frames that are too long. The fossil fuel consumption rates could also drop, if alternative fuel sources become available.