Personally, I have never taken any steps to protect the camera from condensation, and I've had the same camera for about 7-8 years without any signs of damage (although it is a 7D which is a pretty solidly build camera). The only precaution is that I don't change lenses immediately after I get inside.

Also, just leaving the camera in the bag will provide some form of protection. Although not as efficient as a sealed plastic bag, it could be sufficient.

Condensation happens when you bring a cold object to a hot environment. People wearing glasses notice this whenever they get inside from the cold.

The reason is that hot air allows for higher humidity, i.e. it can contain more water molecules (steam) than cold air. So when the temperature suddenly drops around the cold object, the water molecules form water droplets (vapor) which will condensate on the object.

So the ideal time to wrap up the camera is when you are done shooting for the day (remember to take the memory card out first, if you want to process images as soon as you get inside).

Putting the camera in a plastic bag while inside produces the opposite effect. You are capturing air with high humidity inside the bag, and when you bring it outside, and the temperature drops, the water will condensate inside the bag.

I think it's a bad idea to leave the camera in a plastic bag at all times, as any condensation that has been trapped inside the bag will not get out.