From BP Oil:

"World proved natural gas reserves at end-2011 were sufficient to meet 63.6 years of production"

That's at current rates. If we were to switch to an exclusively natural gas infrastructure, this number would of course drop sharply. Natural gas is an appealing alternative, but there just isn't enough of it. And before anyone whips out the old "they'll just find more" canard - not really. The primary reserves of natural gas we're tapping now have been known about for a long, long time, and there are few new discoveries in the pipeline. A similar situation exists with Petroleum.