I have a MBA 2013 i7, it is always in a incipio transparent case, and it only has the fan on when it has been on the bedsheets for 20 minutes.
It is usually ice cold for the first 30 minutes of use, then the palmrests warm up a bit.
As the case radiates heat out from the back of the underside, it may get warm in summer if you wear a tshirt, but is not likely to be too hot unless you're using it somewhere warm.