There is an incredible amount of resource out there. This is a map I posted earlier regarding tight gas.

Shale gas resources are just as widespread. Even coal bed methane is theoretically very widespread, since veins that are too deep to mine can possibly be used.

These resources will certainly vary in quality from one part of the county to another. Originally, none of it could be extracted profitably. Companies have chosen what appear to be promising sites, and gradually made incremental progress on being able to extract the natural gas profitably and in reasonable quantity. It is yet clear how much of this gas can be extracted profitably, since this will depend on how much technology can be adapted to extract gas in different formations. In some formations, the gas may be so "dilute" or bound so tightly that nothing can be done to profitably extract it.

One of the issues in determining gas reserves is how closely wells can be spaced. The original spacing (I believe) was one well to 80 acres. This is now coming down to one well to 40 acres in some places, and even one well to 20 acres a few places.

In my visit to BP's Wamasutter WY location, BP said that it has produced about 3 trillion cubic feet of natural gas since 1977. This represents less than 20% of the resource available in BP's portion of Wamsutter field. We were told that gas wells from the 1970s are still flowing.

If we had unlimited resources (including no peak oil), I can imagine a scenario where unconventional gas could keep producing at a high level for 20 or 30 years, because rigs could be moved on to new locations, as old locations became exhausted. In the real world, I doubt that this is will happen.