What is the ruling on not invoking blessings upon the Prophet (peace be upon him) immediately after hearing his name?

Scholars have differed regarding the obligation of sending blessings upon the Prophet (peace be upon him) when he is mentioned. Some have maintained its absolute obligation every time his name is mentioned, citing hadiths that contain warnings for those who do not send blessings upon him. Others have held that it is recommended (mustahabb), or obligatory only once in a lifetime.

As for those who assert the obligation of sending blessings upon him every time his name is mentioned, it is incumbent upon one to do so immediately without delay. If a long interval passes, its time is lost. However, if the interval is short, there is no harm. It is recommended for a lover (of the Prophet) to send blessings upon him immediately upon his mention.