When should CSS font-family value use quotes?

You only need quotes when the font itself has a space such as "Times New Roman".

Arial does not need quotes, but some people use quotes such as "Arial" to be more consistent. It is simply personal preference.

Seen in Justin's below comment: font-family: times new roman; works without quotes (JSFiddle).

You can call your new @font-face using font-family: 'entypo'; as you would normally expect. (link)