For a very long time. The natural gas infrastructure uses natural gas generators to run the pumps that pressurize the lines and the meters are 100% mechanical. There's no reliance on the electrical grid. I lived through Northeast Blackout of 2003 (visible from space, the extent was so great) and Hurricane Sandy (power was out for a week) and in neither case was there an interruption to natural gas service. In my 40+ years, I don't ever recall the natural gas service ever going "out".