Is it better to set aside a percentage of money specifically for charity, or to give to charity whenever the opportunity arises?

It is possible and preferable for a Muslim to set aside a portion of their wealth regularly for charity; this is an act of goodness and perseverance in it, and it is better than discontinuous charity, as the Prophet, peace and blessings be upon him, said: "The most beloved deeds to Allah are those that are most constant, even if they are small." Imam Al-Nawawi clarifies that the constancy of a small amount makes obedience continuous through remembrance, mindfulness, sincerity, and turning to Allah.