After serious thinking VisionSet wrote :
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!

The larger the diameter of the pipe versus the length, the more easily
will the gas come out to be replaced by air, especially if the pipe
slopes up to the cut end. I would guess at two or three minutes for it
to be clear of gas and safe to solder a two meter length.

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Regards,
Harry (M1BYT) (L)
http://www.ukradioamateur.org