Some people believe that it is obligatory to invoke peace and blessings upon the Prophet (peace be upon him) once in their lifetime and anything that exceeds this is Mustahab (desirable).
(Part No. 3; Page No. 7)
Invoking Allah’s peace and blessings upon the Prophet is Fard (obligatory, based on a definitive text), for Allah’s Command in His Saying (Exalted be He):
O you who believe! Send your Salât on (ask Allâh to bless) him (Muhammad صلى الله عليه وسلم), and (you should) greet (salute) him with the Islâmic way of greeting (salutation i.e. As-Salâmu ‘Alaikum ).
The basic principle concerning the imperative mood is that it denotes obligation or duty. Since the command in the Ayah does not denote repetition, obligation (to do this) must be once in a lifetime and recurrence is desirable, based on the Hadiths mentioned in this regard indicating the desirability of doing so (i.e., invoking peace and blessings upon the Prophet), except at the places specified by Hadiths as being obligatory therein.