Some things you might like to consider (Rofrost):
1. The Rothenberger instructions suggest waiting until temperature shows -30. Takes 15 to 20 minutes on 15 mm pipe.
2. I personally wouldn't solder closer than 450 mm (18") to a freeze plug.
3. If using on plastic pipe, tie the freezer head on with a bit of string as well. Plastic can deform as you tighten the head, leaving it a bit loose.
4. If freezing close to a compression fitting, get all the water between freezer head and fitting out as soon as the plug forms. Otherwise the residual water freezes and makes it difficult to get the fitting back together before defrosting.
5. Wear gloves. I have, only once, got mild frostbite from struggling to get a frozen pipe back in place with the freezer head still attached.