sleeping bag in torres del paine

I was there in mid-December in 2002 - I used a 20 degree bag and was comfortable. Of course weather conditions vary from year to year, but you're never camped at very high altitude and generally at that time of year the issue is more likely to be extreme wind, rather than extremely low temperatures. A 10-degree F bag should be fine.

I use a 0 degree Celsius bag and don't have any problem.

It probably depends on whether you are doing The W hike or The Circuit hike. On "The W" hike a 10 F sleeping bag would be fine. Remember too that a tent keeps the temperature inside the tent warmer and is protected from the wind.

Clarification: I used my 20F on the Circuit - I was never cold. I suspect that he coldest camping areas would probably be Los Perros (higher up than other camp sites) and Refugio Grey area if the wind is blowing off the glacier.