How much natural gas does the United States have and how long will it last?

EIA estimates that there are 2,203 trillion cubic feet (Tcf) of natural gas that is technically recoverable in the United States. At the rate of U.S. natural gas consumption in 2011 of about 24 Tcf per year, 2,203 Tcf of natural gas is enough to last about 92 years.

1Technically recoverable reserves consist of "proved reserves" and "unproved resources." Proved reserves of crude oil and natural gas are the estimated volumes expected to be produced, with reasonable certainty, under existing economic and operating conditions. Unproved resources are additional volumes estimated to be technically recoverable without consideration of economics or operating conditions, based on the application of current technology.

http://www.eia.gov/tools/faqs/faq.cfm?id=58&t=8

and much longer worldwide...

http://reason.com/blog/2011/01/20/what-energy-crisis-natural-gas