How long does it take for a little natural gas to safely clear out of a house?

Okay nobody is in chat and everyone has a life this friday night except for my girls and I so I am asking here in hopes someone can give advice.

My girls and I are outside in the cold because we came home to a house that lightly smelled of natural gas.

On investigation one of the knobs on the stove top was slightly on.

The gas smell was not overwhelming. I did not smell it at all but one of the girls did.

So we opened up every window, did not turn on any lights or fans and are camped out in the great outdoors of the front porch.

So how long should we stay out in the damp cold? It has been a half hour.

I cannot smell gas anymore but is there any kind of set time frame?