I wish theoildrum was still running, since they devoted a lot of intellectual effort into exploring questions like this in great detail. For oil you need the Hubbert Curve: http://theoildrum.com/node/7072

Remember that depletion is asymptotic, so a possible answer is "never"; extraction just gets smaller and smaller. Individual wells work like this anyway, what comes out is an oil/water mixture at gradually reducing pressure. At some point it's uneconomic to pump and filter it, so it's capped off when not empty.