What is the minimum frequency for performing offensive Jihad (fighting)?

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The minimum obligation for Jihad is once every year. This frequency is established because the Jizya (poll tax) is incumbent upon the people of protection (Ahl al-Dhimma) annually, and since Jizya is a substitute for military support (Nasra), its substitute, Jihad, must also be performed annually once, absent a valid excuse. Valid excuses include weakness in Muslim numbers or materiel, waiting for reinforcements, obstacles on the path, lack of fodder or water, or anticipating the enemy's inclination toward Islam if fighting is delayed. A truce (Hudna) may be made if the greater interest dictates postponing fighting, as the Prophet peace be upon him reconciled with Quraysh for ten years and postponed fighting certain Arab tribes without a truce. If necessity demands fighting more than once in a year, it becomes obligatory because it is a collective obligation (Fard Kifaya), requiring what necessity demands.