What are the times when one should not invoke blessings upon the Prophet, may Allah's peace and blessings be upon him?

Praying upon the Prophet (peace be upon him) is a well-established act of remembrance for which there is encouragement. Whoever prays upon him once, Allah will pray upon him ten times. Therefore, one should frequently do so due to the great reward entailed therein. It is only prohibited at times when dhikr (remembrance of Allah) is prohibited, such as when entering the lavatory.