answer:I would think that a half hour is fine, especially if you don’t smell gas any more and you know for certain that the cause was your gas knob left on. But don’t just take my word, I’d suggest you call the gas company and ask to be safe. They might say “yes, it’s safe now,” or they might dispatch a truck just because the rules say they have to on any report of a gas leak, which is probably a good idea anyway…