Natural gas is a little lighter than air and a smallish molecule. I'd say an hour was more than enough for a couple of metres of 22mm.
Don't really want to go bang, when I get the blow torch going.
Plumber I once new said he generally cut, then lit the end!! I guess that's fairly safe if you can point the cut end into open space or against some masonary and you do it immediately before an explosive gas/air mixture has chance to develop. I don't think I'LL bother though!