In this case, "hora" means "time" and it needs no article. Notice that you don't have the article in English either. "É hora de" is an idiomatic expression, it is used to mean "it's time of" or "it's time for".
There is another identical expression (used more in Portugal than in Brazil) that is "Está na hora do almoço". This one uses the article ("na = em + a") because it uses the verb "estar" instead of the verb "ser".