When is it better to wear long sleeves?

It is better to wear long sleeves when it is cold enough outside that exposed skin would be uncomfortable with the temperature. Long sleeves are a matter of personal preference in temperature and style, but if the skin of your exposed arm while wearing a T-shirt is uncomfortable, you should probably wear a long sleeve instead. Also, you may choose to wear long sleeves when it is not as cold out as a fashion statement, which is all a matter of personal preference.